True-red non-retinal tetrachromacy (TRNRT), respectively dichoptic true-red tetrachromacy, represents an augmentation of human binocular trichromacy toward a functional form of tetrachromacy, engineered through dichoptic spectral filtering and specifically trained binocular fusion of differing color signals received by each eye. This article demonstrates how bifurcating (i.e. splitting) the long-wavelength (L) cone sensitivity of a normal binocular trichromat into two mutually exclusive, discrete spectral wavelength ranges (L–: approx. ≤615 nm; L+: approx. ≥615 nm) across both eyes induces a moderately to strongly functional tetrachromatic visual system, enabling trained subjects to perceive a four-dimensional color space inclusive of a selection of dichoptic colors and hues—such as simultaneous red/green, red/yellow, and black/yellow combinations—normally imperceptible under and largely unaccounted for in classical trichromatic models. Based on case studies and computational spectral and dimensional modeling, as well as tests for training and determining the functionality of this dichoptic tetrachromacy, I demonstrate that trained observers can achieve stable binocular fusion and integration of these normally "impossible" dichoptic colors, mitigating and eventually overcoming binocular rivalry through neuroplastic adaptation.

TRNRT introduces a four-dimensional color space model, practically achieved through a pair of glasses with a specific mutually exclusive optical filter pair, defined by distinct one-dimensional subspaces for luminance and saturation, and a two-dimensional subspace for hue. This framework dimensionally expands conventional trichromatic color theory by introducing a fourth virtual but effective interocular dimension of color, demonstrating that subjectively novel color qualia can be stably perceived through dichoptic colors; necessarily in connection with sufficient and adequate training. 

This article is intended to serve a dual purpose: First, it provides a foundational resource for augmenting human binocular trichromacy toward dichoptic tetrachromacy using engineered chromatic redundancy disruption, and for understanding the full implications of this enhancement. Second, this model acts as a resource for advancing our understanding of human retinal tetrachromacy, because both retinal and non-retinal (i.e. dichoptic) tetrachromacy lead to a similar dimensional expansion of color space into a 4th dimension despite differing in color qualia and sensitivity of their effective cone types.

Classically, tetrachromacy is understood as a purely retinal phenomenon. In this conventional view, even if a binocular tetrachromat—whether human or animal—were to lose one eye, their visual system would retain its tetrachromatic nature, as the necessary photoreceptor diversity is intrinsic to the retinal mosaic itself. However, the conceptual framework underlying True-Red Non-Retinal Tetrachromacy (TRNRT) challenges this monocular limitation by proposing a novel mechanism: the generation of functional tetrachromacy through dichoptic means. Here, 'dichoptic' describes a visual state where each eye is sensitized to a distinct, mutually exclusive range of wavelengths, thereby breaking the spectral and chromatic redundancy typically found in binocular vision. This bifurcation of binocular color vision can either be innate or achieved later in life through a specialized pair of mutually exclusive spectral filters.

To illustrate this, consider a theoretical model involving a subject born with asymmetric congenital color vision:

Suppose the first eye is protanopic, exhibiting a sharp spectral cut-off beyond 615 nanometers. It retains normal S-cone function, but possesses M and L cone responses that quickly decrease in functionality after 615 nanometers and otherwise remain normative; rendering long ("deep red") wavelengths indistinguishable from black. 

Conversely, the second eye is red-monochromatic, sensitive only to wavelengths above 615 nm. 

In isolation, neither eye possesses a novel color dimensionality; each operates within a limited subset of standard trichromatic space. 

The critical question, however, arises when these inputs are integrated, when the brain has to fuse these two different perspectives into a single coherent image: Does the subject perceive a color deficient or otherwise disjointed world; a relatively normal trichromacy; or does the neural synthesis of these distinct interocular signals yield a visual experience superior to normal trichromacy?

When analyzed in a binocular context, this anomalous arrangement suggests a capability that strictly deviates from standard trichromacy. In the following sections, I demonstrate—through experiential, experimental, quantitative, qualitative and simulative methods—that this specific form of dichoptic processing does not merely simulate tetrachromacy theoretically but generates a strongly functional tetrachromatic vision; theoretically stronger than the "strongly" functional retinal tetrachromacy some women are believed to very rarely possess. By training binocular fusion of a specific set of dichoptic colors (i.e. binocular combinations of two specific sets of normal trichromatic colors), the observer can resolve a four-dimensional color space, distinguishing upwards of 100 million colors (100⁴) and perceiving a two-dimensional sphere of unique hues that remain inaccessible to the standard trichromatic observer.

True-red non-retinal (chromatically-less-redundant) moderately to strongly functional tetrachromacy, or true-red non-retinal tetrachromacy (TRNRT) represents a binocularly mediated, perceptually transformative advancement in human binocular color vision. At its core lies the strategic disruption of chromatic redundancy inherent to binocular (trichromatic) vision. This is achieved through a pair of inversely transmissive optical filters, engineered to isolate distinct long-wavelength (L-cone) sensitivity ranges across the eyes. The first eye’s filter shifts L-cone sensitivity to shorter wavelengths (L-: approx. ≤615 nm), while the second eye’s filter isolates sensitivity to longer wavelengths (L+: approx. ≥615 nm). This dichoptic division generates two virtual L-cone subtypes, creating mutually exclusive spectral perception across the eyes. The resultant system enables hybrid chromatic integration: retinal (intraocular) mixing of S, M, and L- cones in the first eye, and non-retinal (interocular, binocular or dichoptic) fusion of the L+ signal (in different brightness levels and a single hue quality) with the protanomalous trichromacy of the second eye. 

TRNRT introduces a novel (S/M/L- vs.) L+ opponency channel, a fourth chromatic dimension orthogonal to classical trichromatic pathways. Ordinarily, untrained observers initially experience binocular rivalry, where conflicting inputs from each eye trigger alternating suppression of colors in varying patterns and intensities. However, through structured neuroplastic training, it has been demonstrated that observers can achieve binocular fusion—a cortical process that synthesizes dichoptic signals into (more) stable, coherent, singular and unique (and contextually non-trichromatic; here: tetrachromatic) color percepts (discussed but only superficially tested by: Simmons D. R. (2025)). This enables the perception of normally "impossible" color combinations, i.e. dichoptic colors, such as simultaneous red/green or black/yellow combinations, which transcend trichromatic limitations. 

Critically, these colors and hues are not entirely novel but emerge from the brain’s reinterpretation of binocularly mixed differing trichromatic inputs, reframing them as subjectively distinct qualia that are incomparable to normal trichromatic color qualia. Subjects trained in TRNRT report subjectively novel color experiences for its entire 4-dimensional color space despite dichoptic tetrachromatic colors being an interocular mixtures of normal trichromatic colors.

TRNRT induces a four-dimensional (4D) color space, structured across luminance, saturation, hue, and the novel S/M/L- vs. L+ stereoscopic opponency axis which expands the subspace of hue into two dimensions. Within this 4D framework, the chromaticity subspace occupies three dimensions, while the hue subspace maps uninterruptedly onto a 2D plane—a topological constraint arising from the spherical organization of hues in a tetrachromatic system. This spherical model supports 14 distinct main hue categories (excluding the achromatic point), each encircled by a variable-radius continuum of neighboring hues [cf. Lee, Jessica. et al. (2024)]. This contrasts starkly with trichromacy’s linear adjacency model, where hues transition between two neighbors (e.g.: red → orange → yellow). This tetrachromatic system grants access to a fourth chromatic axis ("true-red" or Capsaicine), a deep spectral red (almost) indistinguishable from other long-wavelength reds to normal trichromats.

TRNRT’s functional superiority stems from its strategic targeting of the high-sensitivity L-cone type, diverging from prior efforts like Gundlach et al.’s (2017) S-cone based approach. By splitting the L-cone’s broad spectral range, TRNRT optimizes discriminability in the red-yellow-green spectral region—a critical advantage for tasks in daily life. Furthermore, its engineered filters and cortical fusion protocols evoke neuroplastic adaptation, training observers to reinterpret dichoptic signals as unified and distinct perceptual experiences. This positions TRNRT not only as a sensory augmentation tool but also as a paradigm and resource for studying neural plasticity in sensory systems. 

Developed by Kilian-Roy Lachner, B.A. at the University of Bayreuth [B.A. in Media Studies (completed: 2017-2025); M.A. in Computer Game Studies (started: 2025)], TRNRT constitutes a foundational innovation in perceptual engineering. While previous work on the conceptual framework of binocular redundancy disruption already exists [cf. Gundlach et al. (2017)], TRNRT’s L-cone spectral bifurcation and cortically integrated fusion protocols in combination with the interactive visualizations of the resulting tetrachromatic space represent a distinct intellectual leap. Unlike incremental improvements to existing technologies, TRNRT establishes a novel method for generating, understanding and communicating tetrachromatic perception, protected as intellectual property. Its measurable success and functionality underscores the viability of binocularly disruptive approaches to visual enhancement, bridging media studies, computer science, neuroscience, and engineering. 

True-red non-retinal tetrachromacy transcends classical trichromatic limits, demonstrating that engineered spectral filtering, coupled with neuroplastic training, can expand human perceptual boundaries. Its implications span multispectral imaging, adaptive human-computer interfaces, empirical studies of qualia in augmented sensory modalities, enhanced color discriminability, as well as tetrachromatic color theory and art, for example. As both a technological invention and a neuroscientific paradigm, TRNRT is a novel approach to functionally simulate tetrachromacy, challenging conventional models of color vision while illuminating the brain’s remarkable capacity for sensory reinterpretation.  

To ensure methodological clarity, the following nomenclature is established:

• True-Red Non-Retinal Tetrachromacy (TRNRT): Refers to the chromatically less redundant, binocularly mediated tetrachromatic system described in this study. The term emphasizes its engineered disruption of spectral redundancy and non-retinal perceptual integration.

• True-Red (Tetrachromacy) Glasses: Refers to the custom optical apparatus containing the dichoptic filter pair, which induces TRNRT by bifurcating L-cone sensitivity of normal trichromats.

• True-Red Tetrachromatic Colors/Hues: The are dichoptic colors largely exclusive to TRNRT observers (e.g., a dichoptic red/green color experience).

This article distinguishes TRNRT from retinal tetrachromacy and anchors its novel perceptual framework.

All figures, tables, and computational programs presented herein—excluding explicitly cited external works—were developed de novo by the author. These visualizations are designed to:

• Illustrate the spectral transmission profiles of the dichoptic filters.

• Map the 4D color space of TRNRT; for example its spherical and 2D hue topology.

• Demonstrate binocular fusion dynamics through stereo demos.

This original content ensures empirical reproducibility and provides a foundational reference for future research.

A critical distinction is drawn between trichromatic and tetrachromatic color terminology:

• (Normal) Trichromatic Color Names: Describes hues perceived under standard human vision (S/M/L cones). For example, colors like red, yellow, green, cyan, blue, magenta, white and black.

• Tetrachromatic Color Names: One system that assigns unique identifiers to TRNRT-specific hues, reflecting their combinatorial cone activations as well as their trichromatic origins. Another system that gives a set of discrete TRNRT hues (and thus colors) a unique name.

• "Hexachromatic (Hexadecimal) Code": The combination of two hexadecimal codes. In true-red tetrachromacy, the first part indicates the first eye's slightly anomalous trichromatic vision (mild protanomaly) and the second part indicates the second eye's deep red monochromatic vision.

The distinction between trichromatic and tetrachromatic color names is necessitated by TRNRT’s expanded color space, where hues like Capsaicine (true-red/black) or Citrine (true-red/green) defy trichromatic linguistic frameworks. Standard color terms (e.g., "yellow") are insufficient and cannot be used to refer to tetrachromatic colors, because tetrachromatic hues occupy distinct perceptual and spectral coordinates.

To approximate TRNRT’s dichoptic colors, stereo demos are provided, separating chromatic components for each eye:

• Frist Eye: Approximately displays the anomalous trichromatic cone activations (S/M/L- mixtures).

• Second Eye: Approximately displays and isolates the L+ cone activation.

By employing cross- or parallel-viewing techniques, observers can binocularly fuse these dichoptic inputs, approximately simulating TRNRT’s novel color experiences. This method bridges theoretical constructs (e.g., 4D color space) with subjective experience, offering readers with normal trichromacy an empirical window into tetrachromatic perception. For simplicity, the slightly protanomalous color vision of the first eye is depicted in normal trichromatic colors. This closely matches the color qualia induced be the true-red glasses, but it is not identical.

True-red non-retinal tetrachromacy (TRNRT) is based on four functionally distinct cone channels—S, M, L-, and L+—that collectively generate a functional four-dimensional color space. These channels yield a diverse plane of spectral and mostly non-spectral hues [e.g., Capsaicine (true-red/black) or Citrine (true-red/green)], with an additional achromatic (i.e. perceptual white) point when all cones are maximally and uniformly stimulated. While such a system parallels aspects of retinal tetrachromacy in individuals believed to have a fourth mutated M' or L' cone type (e.g., the type associated with "cDa29," which Concetta Antico is hypothesized to possess, as tested by Jordan, G. et al. (2010) and Jordan, G. et al. (2019)), TRNRT differs in both its methodology (i.e., engineered L-cone bifurcation via dichoptic filters) and in the degree to which it confers tetrachromatic functionality. Because the novel L- cone remains proximal in sensitivity to the M cone and both L- and L+ subtypes share a similar "primary" color quale, the overall enhancement is moderate relative to hypothetical scenarios involving a more divergent fourth cone type in the ultraviolet or infrared range that evokes a more unique "primary" color qualia (compared to L- and L+) as well as a more unique peak sensitivity (compared to M and L-).

A central factor in TRNRT’s utility lies in the spectral composition of environmental lights and materials. Pure "yellow" signals (yellows that aren't a mix of red, yellow and green), for example, are relatively rare in natural settings, with most stimuli in the red-yellow-green range arising from blended or broadband spectral distributions. The practical value of any tetrachromatic system thus hinges on encountering stimuli whose spectral bandwidth and reflectance properties meaningfully engage the extra cone channel. In TRNRT, the L- cone lacks long-wavelength sensitivity. This produces a greater amount of metameric or near-metameric matches with black when compared to a normal trichromatic L cone, reducing the visibility of wavelengths longer than 615 nm. By contrast, the L+ cone, anchored at longer wavelengths (≥615 nm), exhibits more pronounced separation from M, offering increased color discrimination of M and L+ under appropriate spectral conditions.

When the environment predominantly excites only the S, M, and L- cones, observers (i.e. normal trichromats wearing the true-red tetrachromacy glasses) effectively exhibit a mild protanomaly, experiencing reduced sensitivity to deep reds. Conversely, filtering conditions that engage only S, M, and L+ yield an anomalous trichromacy closer to the typical human norm, albeit lacking in the 550-615 nanometer region. Interestingly, a scenario stimulating only the M, L-, and L+ cones also constitutes a form of trichromacy, yet it would be more weakly functional than standard trichromacy due to the overlap of the M and L- spectral cone sensitivity curves and the similarity of the L- and L+ channels' ideal primary color qualia.

From a practical perspective, a subset of TRNRT-generated color experiences—particularly those heavily reliant on narrowband or quasi-monochromatic and multi-spectral signals (3 or more distinct narrow wavelength peaks)—remain relatively rare in everyday life. Much like magenta, which is scarcely found in nature because it lacks a direct singular spectral counterpart, many true-red tetrachromatic hues are composites of multiple overlapping wavelengths rather than single, pure wavelength peaks. Since modern lighting and displays are largely optimized for trichromatic perception and natural scenes' lights colors exhibit predominantly broad ranges of illumination and reflectance (because "most natural reflectances track the 1D arc of the spectral locus in the 2D [tetrachromatic] hue sphere"), some of the distinctive colors and hues of TRNRT’s 4D color space and 2D hue subspace do not commonly and easily arise. Nevertheless, TRNRT vividly demonstrates how spectral engineering can unlock new perceptual color categories when the interplay of cone activation is systematically altered.

True-red non-retinal tetrachromacy (TRNRT) yields a marked increase in the number of unique cone-activation patterns relative to standard trichromacy, thereby enriching both the quantity and quality of perceivable hues. Under normal trichromatic vision, there are seven fundamental cone-activation states (including the achromatic point)—(S), (M), (L), (S/M), (S/L), (M/L), and (S/M/L)—plus a total cone inactivation (K), which collectively define the principal axes of the trichromatic color space. All other color and hue experiences arise from mixtures of these states, but the seven unique activations stand out as canonical references.

In TRNRT, however, the bifurcation of the long-wavelength cone adds the L- and L+ subtypes, expanding the repertoire to fifteen unique cone-activation patterns (including the achromatic point)—(S), (M), (L-), (L+), (S/M), (S/L-), (S/L+), (M/L-), (M/L+), (L-/L+), (S/M/L-), (S/M/L+), (S/L-/L+), (M/L-/L+), and (S/M/L-/L+)—alongside total cone inactivation (K). Crucially, TRNRT also introduces three-fold cone combinations that do not appear in normal trichromacy; for instance, (S/M/L-), (S/L-/L+), or (M/L-/L+). Under standard trichromacy the three-cone mix S/M/L+ collapses to perceptual white, for example, whereas in TRNRT producing the perception of white further requires the stimulation of the L- cone; i.e. simultaneous stimulation of all four cone subtypes.

Although retinal tetrachromacy involving an additional mutated M' or L' cone (as documented in research on individuals such as "cDa29") can exhibit more pronounced and stable perceptual contrast—owing to a theoretically greater functionality and stability of intraocular cone type mixes—TRNRT still achieves a moderately to strongly functional and distinct four-dimensional color space. Here, despite its similarity in color quality to the the L- cone, the L+ channel remains fully distinct from the S, M, and L- channels and introduces qualitatively new chromatic experiences in a binocular context. Strong functionality of TRNRT is likely to be a skill that subjects have to specifically train for.

Part of TRNRT’s distinctiveness arises from the fact that the L+ channel of the first eye is fully orthogonal to and distinct from the S, M, and L- cone channels of the second eye (i.e., there's almost no overlap of the cone type's curves), even when the "primary" color qualia of both the L- and L+ cones are similar. In practice, the fourth color axis manifests from the red-monochromatic L+ spectrum of the first eye mixing with the slightly protanomalous S/M/L- 3D color space of the second eye. This mixture is sufficiently orthogonal to trichromatic processes that it produces genuinely novel, non-retinal (i.e. interocular or dichoptic) hues that expand the color space beyond the reach of standard human color vision into a fourth dimension.

Physiologically, the brain processes six color channels, labeled S1, S2, M1, M2, L1, and L2—where "1" and "2" denote the left and right eye, respectively. However, because of the chromatic redundancy and similar spectral sensitivity of both eyes' color vision, each eye approximately sees the world in the same colors, although both eyes' cone types are technically distinct. Only by disrupting this chromatic and spectral redundancy we can access this potential for human hexachromacy in normal binocular trichromats.

A series of videos explores the concept of true-red non-retinal tetrachromacy (TRNRT) and visually demonstrates the functionality of the dichoptic true-red tetrachromacy glasses in moving pictures. Among these, This is How I Turned Myself Into A Tetrachromat (VR) by Ooqui (Lachner K. (2024)) emerges as the most comprehensive video currently available, presenting an in-depth exploration with many visualizations of how TRNRT can be induced, understood and visualized. Yet, every one of the videos in this serious is recommended for a more complete and robust comprehension of TRNRT. These videos are tailored for Virtual Reality (VR) or cross-/parallel-viewing techniques—two approaches that approximate the tetrachromatic color experiences produced by the true-red tetrachromacy glasses. 

In the first video, How to Achieve True Red Cone Tetrachromacy (VR), the showcased glasses omit the bandpass 360–580 nm (FGB39) filters necessary for the second eye, resulting in only partial TRNRT functionality. The subsequent video, This is How I Turned Myself Into A Tetrachromat (VR), rectifies this shortcoming by incorporating the missing filter(s), thereby granting a more complete expression of L+ channel separation and providing viewers with a closer approximation of genuine four-dimensional color vision.

Generally, the latest videos in this series (here at the bottom of the following list) correct mistakes made in older videos and have the newest findings of my research incorporated.

The cone types' spectral sensitivity curves of the True-Red Tetrachromatic Cone Responsivity (Bell Curves) program are an approximation. This article contains more accurate line diagrams for TRNRT.

Two of the most famous examples of supposed strongly functional female retinal tetrachromats are Concetta Antico and Maureen Seaberg. Because of their genetic predisposition these two women have a high chance for tetrachromacy (Jordan G. et al (2019), Jameson K. A. (2009) & Jameson K. A. et al. (2020)). In the following it is assumed that these two and other specified women possess a strongly functional tetrachromacy, or at least a degree of functionality that allows for tetrachromatic behavior and color discriminability. The way that Antico and Seaberg describe the colors they see is extraordinary. However, both of them are predominantly using trichromatic color names to describe their tetrachromatic colors. In the following you can see a few examples of how these tetrachromats describe the colors they see. The following quotes focus on the tetrachromatic color description of retinal tetrachromats.

Although this is only a portion of the information that's available online on how retinal tetrachromats translate their color experiences, these examples are enough to highlight a significant problem when it comes to describing and naming tetrachromatic color experiences.

This problem has also been mentioned by Lee, Jessica. et al. (2024) in their study's subitem "4.3 Tetrachromats are Surrounded by Color Blindness":

"One of the earliest tells that a child is color blind occurs during art class. Since hues like green and pink are confused in red-green color blindness, a color blind child often paints the grass pink, and a rose green, [...]. When a trichromat looks at the image, they perceive the colors as discordant with reality. To get a sense of the color experience of a tetrachromat, imagine what it would be like if you were a trichromat in a world built for dichromats. The paint box would lack the colors to paint a rainbow. Many of the hues painted in the world would appear incongruent with reality to you, because the artists were color blind. On the other hand, if the only colors that existed in the world were blue and yellow, if the world was entirely dichromatic, these incongruous colors and lack of rainbows would seem normal to you. Returning to the case of tetrachromats living in our reality, the defining characteristics of their color experience is living in a world colored entirely by the color blind (trichromats), who have manufactured a world filled with trichromatic colors, but few tetrachromatic colors outside their color blind gamut. In addition, the natural world lacks tetrachromatic colors outside the trichromatic gamut (e.g. projecting hyperspectral images [...] into tetrachromatic colorspace shows that most natural reflectances track the 1D arc of the spectral locus in the 2D hue sphere). Similar to a trichomat living in a dichromatic world, a tetrachromat in our world today may rarely encounter colors that they would view differently to trichromats. If we alter our manufacturing systems to produce colors for tetrachromats, a tetrachromat viewing these colors might have a experience akin to a trichromat living in a dichromatic world and one day seeing a complete set of paints for the first time."

Tetrachromats are mostly surrounded by trichromats. As such, most of the intentional human design work was made with and for trichromatic vision. This doesn't just include man-made material and light colors, but also color names. The above quotes of female tetrachromats unanimously demonstrate that it's difficult for these women to convey their color experiences through language. They're using trichromatic color names or compounds thereof to indicate their personal tetrachromatic color experiences. However, using trichromatic color names for tetrachromatic colors is inherently problematic.

In the following there's a focus on hue, because the brightness and saturation of hues stay identical across all color vision dimensions above monochromacy and only adjust indirectly to the dimensionality of hue. Identically to di- and trichromatic hues, any tetrachromatic hue can only vary in brightness and saturation.

To understand why describing tetrachromatic hues with trichromatic color names is an impossible task, let's consider the following equivalent but lower-dimensional example: 

A trichromat trying to describe the 1-dimensional hues of trichromacy to a dichromat, e.g. a protanope, with only the two dichromatic, 0-dimensional hue names, yellowish and bluish, is practically impossible. For example, by converting the following quote (taken from the above quotes in 7.4 Naming System for True-Red Tetrachromatic Hues) by Concetta Antico to one dimension of color lower, the problem becomes highlighted:

• Original Quote (tetrachromatic hues described with trichromatic hue names): When Concetta Antico looks at a leaf, she sees much more than just green. “Around the edge I’ll see orange or red or purple in the shadow; you might see dark green but I’ll see violet, turquoise, blue,” she said. “It’s like a mosaic of color.” (Tetrachromatic color description by: Concetta Antico)

• Adjusted Quote (trichromatic hues described with dichromatic hue names): When [a trichromat] looks at a leaf, [they see] much more than just [yellow]. “Around the edge I’ll see [yellow] or [blue] or [lighter blue] in the shadow; you might see [dark yellow] but I’ll see [dark blue], [light blue], [blue],” [they] said. “It’s like a [line] of color.”

This adjusted quote aims to expose the problem of naming tetrachromatic hues with trichromatic hue names. A dichromat reading the color description of the adjusted quote cannot use the presented dichromatic color names as a means to understand or even relate to the color experiences of the describing trichromat. The only new information this dichromat gains from the trichromat's color description is that the trichromat can see more colors where they see fewer. As much as dichromatic hue names are inadequate for describing trichromatic hues, trichromatic hue names are inadequate for describing tetrachromatic hues.

Hue in tetrachromacy is 2-dimensional. As such, we need an adjusted system for locating tetrachromatic hues that's equivalent to the characteristics of a sphere's surface. Whereas it sufficed to radially categorize the 1-dimensional hues of trichromacy with a degree of latitude (0°-360°), in tetrachromacy you need both a degree of latitude and a degree of longitude to describe the location of one of its 2-dimensional hues. For more precision let's define the following terms:

Trititude: The trichromatic hue latitude, called "trititude", describes the trichromatic nature or base of a tetrachromatic hue and is indicated by a "³" (i.e. a small raised 3) directly after a degree value. For example, "60³" refers to the trititude "yellow". But trititude alone is not enough to locate a tetrachromatic hue.

Tetritude: The tetrachromatic hue longitude, called "tetritude", in turn describes the tetrachromatic nature of a tetrachromatic hue and is indicated by a "⁴" (i.e. a small raised 4) directly after a degree value. For example, a tetritude of "60⁴" refers to an equal mix of one of the trichromatic hues with the 4th primary color.

Program 7.3/1 & Program 7.3/2 interactively demonstrate the labeled TRNRT 2D hue sphere/plane. See the respective toggles at the top of these programs to choose which parts of the label should be enabled/disabled.

Using trititude and tetritude as an expanded example to showcase the problem of naming tetrachromatic colors with trichromatic hue names: only using trititude for locating a tetrachromatic hue is impossible. If done nonetheless, it naturally leads to confusion because with only a specification of trititude the tetrachromatic hue exists in a superposition on the line of tetritude. Both trititude and tetritude are essential for describing the 2-dimensional location of a tetrachromatic hue. 

However, the female retinal tetrachromats that have been quoted above in this subitem were all trying to convey their tetrachromatic color experiences with trititude only. Such limited descriptions make the colors they describe sound more real and approachable for normal trichromats, but they can't convey the degree of tetritude of any color. There needs to be a standardization for naming tetrachromatic hues. At a minimum there needs to be a single naming system that can universally be used by any tetrachromat for describing their color experiences. And at best there should be a unique naming system for every differing tetrachromacy.

Unfortunately, there are no official or standardized tetrachromatic color names yet (for human color vision). There are too few functional (retinal or non-retinal) human tetrachromats for a tetrachromatic naming system to have naturally emerged, and too few for them to have gathered to create one. Some tetrachromats might personally use unique tetrachromatic color names to categorize colors, but sharing them with uninitiated trichromats becomes a challenge because they're not only unfamiliar with these terms, but additionally can't distinguish tetrachromatic colors.

In the following I aim to establish reasonable, logical and perception-based standardized tetrachromatic hue names for TRNRT hues. The following naming systems are by no means flawless, but because they introduce new tetrachromatic color names they make tetrachromatic hue naming possible.

The first and fundamental color naming system for (true-red) tetrachromacy is called "tetrachromatic hue flavor" or "4D hue flavor". Just like blue in normal trichromacy can look "cold", red can look "hot" and green somewhere in between of "cold" and "hot", in TRNRT you can correlate each tetrachromatic hue with a specific flavor, flavor profile or taste-based categorization.  For example, a dichoptic true-red/blue can look "bitter", a true-red/green "sour", and a true-red/yellow "hearty" (see: Program 7.3/1 with the toggle "Hue Adjective" enabled). This theme was chosen because it fit the subjective feelings or personal qualia of true-red tetrachromatic hues well across a few individuals who've experienced (interlaced & dichoptic) TRNRT hues before (cf. "Ooqui Sensory Lab" Discord server, on-going unofficial case studies and experience reports in #dichoptic-tetrachromacy). 

The adjective 4D hue flavors of the Programs 7.3/1: Labeled Unfolded Dichoptic 2D True-Red Tetrachromatic Hue Sphere & 2D Hue Ordering Test are still a work-in-progress and may be updated at any time. These adjective hue flavors weren't chosen randomly, there's a perception and subjective experience based structure behind them. This flavor-based hue categorization was first proposed by Discord user "RareBeeph" in 2024 on the Ooqui Sensory Lab Discord server (specifically this post), although in a simpler form with only 6 hue flavors for TRNRT hues.

Program 7.3/1 & Program 7.3/2 interactively demonstrate the labeled TRNRT 2D hue sphere/plane. See the respective toggles at the top of these programs to choose which parts of the label should be enabled/disabled.

Using tetrachromatic hue flavors in conjunction with trichromatic color and hue names, it now becomes possible to describe both the trititude and tetritude of a true-red tetrachromatic hue. For example, a "sour green" is an equal mix of green and true-red; a "tannic cobalt" is an equal mix of cobalt and true-red; and a "galvanic yellow" is a mix of light-yellow and black. In this simple naming system every trichromatic color name is preceded by an adjective that describes that color's tetritude.

If we take the above quote from Concetta Antico as an example again...

• Original Quote (tetrachromatic hues described with trichromatic hue names): When Concetta Antico looks at a leaf, she sees much more than just green. “Around the edge I’ll see orange or red or purple in the shadow; you might see dark green but I’ll see violet, turquoise, blue,” she said. “It’s like a mosaic of color.” (Tetrachromatic color description by: Concetta Antico)

...we can convert it into a more reasonable tetrachromatic color description:

• Adjusted Quote (tetrachromatic hues described with a combination of trichromatic hue names (trititude) and tetrachromatic hue flavors (tetritude)): When [Ooqui] looks at a leaf [with dichoptic true-red tetrachromacy], [he] sees much more than just green. “Around the edge I’ll see [mouldy green] or [sour green] or [vinegary lime] in the shadow; you might see dark green but I’ll see [soapy turquoise], [acidic lime], [earthy yellow],” [he] said. “It’s like a [rainbow times a rainbow] of color.”

Although not perfect, this adjective flavor based naming system for (true-red) tetrachromatic hues now allows anyone who's familiar with these 4D hue flavors to comprehend what colors a (dichoptic true-red) tetrachromat is talking about.

Program 7.3/1 & Program 7.3/2 interactively demonstrate the labeled TRNRT 2D hue sphere/plane (also found with less functionalities in item: 13.3 Complete (True-Red) Tetrachromay 2D Hue Ordering and Discriminability Tests). See the respective toggles at the top of these programs to choose which parts of the label should be enabled/disabled.

Using these true-red tetrachromatic hue names, optionally with the specification of their trititude and tetritude, it is possible to describe the exact location of a hue on the true-red tetrachromatic hue sphere. Instead of a flower "having many shades of red", it is now possible to describe it as a flower that mainly blooms in Capsaicine, but some of its petals are more Appetine and Torrefine, and one or two are even slightly Cuisine and Cherrine, among other hues.

The following Program 8.1/1: True-Red Tetrachromatic Hue Sphere – Locally Trichromatic Hue Circles provides a 2-dimensional surface visualization of the true-red tetrachromatic chromaticity ball (i.e. a solid sphere) in a 3-dimensional structure, which itself is a subspace of the overall 4-dimensional color space. Only the outer surface of this chromaticity ball is visible. Each surface color corresponds to a unique true-red tetrachromatic hue, and any given hue can vary in terms of saturation and brightness. Tetrachromatic hues with varying saturation and brightness levels extend inward and into a 4th dimension of color, beyond our direct view, which makes it impossible to view the whole 4D color space at the same time for 3D observers. By using stereo viewing techniques to superimpose the program’s left and right views you can approximate the binocular fusion technique that reveals how these surface hues relate to one another one a continuous 2D surface. This might be a difficult task for viewers inexperienced in fusing a continuous arrangement of dichoptic true-red tetrachromatic colors. Program 7.3/2: Labeled Dichoptic 3D True-Red Tetrachromatic Hue Sphere & 2D Hue Ordering Test resolves this difficulty by discretely displaying this intrinsically continuous hue sphere.

An optional Show Overlay feature in Program 8.1/1 further clarifies each highlighted true-red tetrachromatic hue by emphasizing its circle(s) of neighboring hues. Significantly, each hue on the continuous 2D surface is surrounded by its own multitude of locally trichromatic hue circles. "Locally trichromatic" here refers to there being more than one distinctly trichromatic hue circle. This observation aligns with insights from Lee, Jessica. et al. (2024), whose Subitem 4.2 (Every Tetrachromatic Hue is Ringed By Its Own Unique Hue Circle) demonstrates that the classic trichromatic model—where each hue is bounded by exactly two hue neighbors—transforms radically under tetrachromacy. In trichromacy, for instance, an orange hue might be surrounded only by two hues, its more reddish and more yellowish variants. Yet in tetrachromacy, that very same stimulus has a continuous circle of distinct neighboring hues and not just two, reflecting the spherical topology of the tetrachromatic hue sphere.

Because each true-red tetrachromatic hue occupies a point on this spherical surface, you can traverse this hue subspace in more complex ways than the respective hue subspace of trichromacy. Regarding hue in 2- to 4-dimensional color vision: In dichromacy there are only 2 unique hues; in trichromacy you can already move around in a circle of unique hues, a 1-dimensional circular and continuous path; and in tetrachromacy you move along the 2-dimensional surface of a sphere of unique hues. Each of these hue subspaces are expanded by the saturation and brightness dimensions. In dichoptic true-red tetrachromacy, this expanded geometry of hue allows for a dimensionally greater hue discriminability and consequently also color discriminability.

In a digital context with idealized primary light and material colors, trichromacy typically supports 1,530 discrete hues, whereas true-red tetrachromacy can theoretically yield approx. 1,170,450 hues (calculation: 1,530 * (255 * 3) = 1,170,450; i.e. three times 255 luminance values times the maximal standard amount of digital trichromatic hues; this number includes the overcounted hues on the two rotation points which results in 1530 duplicate hues). This is not the exact number of true-red tetrachromatic hues, but it's a close estimate. For comparison, the jump from dichromacy to trichromacy is a 765-fold increase in possible hues, and the jump from trichromacy to tetrachromacy is another 765-fold increase.

The subspace of hue in a tetrachromatic system is necessarily two-dimensional—embedded in a higher-dimensional (4D) color space—and cannot be fully captured by the one-dimensional hue model (and color names) that normally suffice for trichromatic vision.

Use stereo viewing techniques to overlap the following images' left and right color squares. Every trichromatic color of the first eye, i.e. its differing S, M and L- cone activation mixes, can be put orthogonal to the different activation states of the L+ cone activation of the second eye (including a seemingly paradoxical red vs. red contrast that's distinguishable in practice thanks to color context). Program 8.2/1: Dichromatic Hue Gradients of Dichoptic True-Red Tetrachromacy shows the multitude of locally dichromatic color subspaces of dichoptic true-red tetrachromacy, with varying brightness and optional saturation levels.

There are 3 additive primary hues in trichromacy; approximately: red, green and blue. Evenly mix these primary colors to get the 3 secondary hues: yellow, cyan and magenta. And evenly mix primary and secondary hues to get the 6 tertiary hues: orange, lime, turquoise, cobalt, purple, pink. Every trichromatic hue only has two hue neighbors, one in each direction of the hue circle. This limits the amount of secondary and tertiary hues to a finite, definable amount.

However, the 2-dimensionality of tetrachromatic hues complicates a finite definition of secondaries and tertiaries and makes them a much more "fuzzy" concept. The addition of a 4th primary color in tetrachromacy (in additive or subtractive color mixing), e.g. "true-red" in TRNRT, leads to a profound change in the definition of secondary and tertiary hues. Lee, Jessica. et al. (2024) correctly state that "every tetrachromatic hue is ringed by its own unique hue circle". As such, each tetrachromatic hue not only has 2 secondary hue neighbors as compared to trichromacy, instead: every tetrachromatic hue is ringed by its own unique circle of secondary hues (as discuss in subitem: 8.1 Examples of the Uniquely Trichromatic Hue Circles of the Dichoptic True-Red Tetrachromatic 2D Hue Sphere). Likewise, there are rings of tertiary hues. Similar to how the concept of hue is 0-dimensional in dichromacy but becomes 1-dimensional in trichromacy, the secondary and tertiary hues are conceptually 0-dimensional in trichromacy but become 1-dimensional in tetrachromacy thanks to the addition of a 4th primary color. 

As an example, a TRNRT hue like Appetine (black/true-red) is ringed by a circle of secondary hues, approximately including: Delicine (true-red/magenta), Thermine (true-red/red), Turmerine (true-red/yellow), Saline (black/yellow), and many other relative secondary hues. These secondary hues encircle the primary color in a specific pattern (i.e. not necessarily circular depending on the color space depiction).

We've already learned that hue in tetrachromacy becomes 2-dimensional. Add to that the remaining two dimensions, saturation and brightness, and a 4-dimensional color space emerges. Saturation and brightness remain constant and only indirectly adapt to the increased dimension of hue after monochromacy. In normal trichromacy a balanced activation of its 3 cone types already generates a perceptual white. Yet, in dichoptic true-red tetrachromacy any balanced activation of 3 cone types will be experienced as a distinct hue experience, color experiences that are as different from perceptual white as the standard hues in normal trichromacy. Because any well-chosen 3-fold cone mix in tetrachromacy necessarily yields fully saturated hues, the interplay of hue and saturation in tetrachromacy is as fundamentally different to trichromatic chromaticity as trichromatic chromaticity is different to dichromatic chromaticity. Dichoptic true-red tetrachromacy has a quirk when it comes to understanding the difference between hue and saturation. Metaline (black/white), which is a balanced S/M/L- mix, might look white to inexperienced viewers. However, because this apparently trichromatic white can be augmented by adding L+ to the mix, it necessarily becomes a hue experience instead of perceptual white. Thus, Metaline (black/white) is a hue experience like Vitamine (black/green) and Acridine (true-red/blue). In dichoptic true-red tetrachromacy perceptual white only emerges when mixing Metaline (black/white) and Capsaicine (true-red/black) to get Hot White (true-red/white).

The following interactive Program 9/1: Trichromatic Hue-Saturation Disc vs. Protanopic Color Space further illustrates these principles. In the trichromatic system, fully saturated hues are distributed along the outer perimeter of a chromaticity disc, with saturation diminishing as the hues approach the achromatic center. By manipulating the Red Amount (%) slider, the conventional trichromatic chromaticity space can be morphed into a distorted but full protanopic color space—depicting every color perceptible to a protanope on an RGB screen. This stands in stark contrast to the standard trichromatic chromaticity disc, which represents only hue and saturation without incorporating variations in brightness. The difference in hue and chromaticity when comparing tetrachromacy to trichromacy is similarly pronounced. The entire 3D color space of normal trichromacy only represent one out of many distinctly trichromatic 3D color spaces in dichoptic true-red tetrachromacy that together form a 4D structure.

Program 10/1: Dichoptic True-Red Tetrachromacy Tetrahedron offers a complementary perspective, depicting how the 3-dimensional chromaticity subspace of tetrachromacy (here: an equilateral tetrahedron) can be unfolded into a plane. In this representation, each face shows dichoptic true-red tetrachromatic hues. Toward the hollow interior you would find the varying saturation levels of these tetrachromatic hues. The three faces that extend into the L+ dimension are alien to normal trichromatic observers, showcasing hues formed by mixing the L+ cone's color qualities with the anomalous trichromatic color qualities. Whereas normal trichromacy effectively restricts observers to a single line of hues along the perimeter of a single tetrahedron's face, dichoptic true-red tetrachromacy brings forth an entire tetrahedron of superficial hue and volumetric chromaticity variation. Each locally trichromatic face is merely one part among many, each populated by additional, distinct hues that only become visible all at once when this 3-dimensional chromaticity subspace is hollowly unfolded.

In the following you can find simple, non-interactive visualizations of the TRNRT hue plane.

These previous figures discretely display dichoptic true-red tetrachromatic hues to make it easier for people to view them who are inexperienced in binocular fusion. However, there are many more intermediate hues and even more colors.

Importantly, although in dichoptic true-red tetrachromacy the L- cone appears to be less functional than in normal trichromacy, as suggested in Table 11.2/3, for narrow wavelength ranges (approx. 1nm-5nm width) this graph depicts wavelength dimming and not hue shift. Under a UQG dichroic cyan filter a narrow 600 nanometer wavelength range (approx. ±3nm) still appears reddish-orange and only becomes dimmed. However, a broader range of intertwined wavelengths as found in a spectrum refracted by a prism might additionally shift in hue for the anomalous trichromatic eye (SML-), depending on the light source used, as the different intensities of overlapping wavelengths adjust to the dimming effect.

This is important to understand because it means that dichoptic true-red tetrachromacy is much more functional than the graph of Table 11.2/3 suggests. In fact, the M and L cones' hue qualities remain the same and only their brightness becomes increasingly dimmed as the transmission of the UQG dichroic cyan filter decreases. As a consequence, in the context of narrow and bright enough primary lights, for example, a 605 nanometer "vermillion" wavelength will still appear vermillion to a dichoptic true-red tetrachromat, meaning that functionality in this range is mainly decreased due to brightness decrease and less due to hue assimilation.

This is also why the following Program 11.2/1: Dichoptic True-Red Tetrachromacy vs. Dichoptic SMQL Tetrachromacy depicts the wavelength specific colors of the anomalous trichromatic eye in normal trichromatic colors and only dims them respective to the reduction in transmission of the UQG dichroic cyan filter.

When attempting to model the transition from normal trichromacy to a functional 4-dimensional tetrachromatic color space—such as in true-red non-retinal tetrachromacy (TRNRT)—a fundamental mathematical problem arises: How do we accurately calculate the number of functional, discrete color steps a biological cone type can produce when its spectral sensitivities overlap with other cones?

to be continued

How do true-red tetrachromatic colors look like through the true-red glasses? Understanding the perceptual qualities of true‐red tetrachromatic colors poses both a conceptual and practical challenge. On one hand, one might document the phenomenon by capturing images of a given color stimulus—first under standard conditions and then through each filter of the true‐red glasses. However, because conventional cameras approximate trichromacy rather than perfectly replicating human (or tetrachromatic) perception, these images must be interpreted with caution. Discrepancies between camera reproduction and subjective human experience are inevitable; therefore, the following visual examples are provided unedited, with clarifications noted where the camera’s rendition diverges from (my) personal observation.

Consider first an RGB (S/M/L+ with a little L- "pollution") light source, which under normal trichromatic conditions appears white because all three cone types are equally stimulated. Figure 12/1a illustrates this RGB light as captured by a phone’s camera. Although trichromats perceive this light as white, a protanope would register a similar achromatic point even if only green and blue components were present.

The true transformation occurs, however, when the true‐red glasses are employed:

These images (Figure 12/1b-1c) reveal that what appears as “white” to standard trichromatic observers is, for a true‐red tetrachromat, a vivid red-cyan (Acy) hue. For the tetrachromatic system to generate a genuine achromatic (white) percept, the cyan component would require a higher contribution of true vermillion light. The resulting red-cyan combination represents a distinct tertiary hue, one that is fundamentally different from any hue generated within conventional trichromacy.

A further illustration of these effects is provided by a real-life example involving various “yellow” hues. Figure 12/2a presents a yellow-orange-red light source viewed with a filter that transmits most wavelengths except true-red. To the naked eye, the light and filter appear nearly identical—a scenario in which the camera fails to capture the same trichromatic colors that are apparent in direct observation.

However, this yellow of Figure 12/2a transforms into a remarkably different binocular hue with the true-red glasses.

Thus, a light source that initially appears uniformly yellow can divide into two distinct tetrachromatic hues—one red-yellow and one black-yellow—each as perceptually disparate as the differences between red and yellow or blue and cyan in conventional trichromatic color vision. Moreover, the true-red tetrachromatic system reveals a rich gamut of hues within the red-yellow-green spectral region—ranging from combinations such as true-red/red (Ar), red/orange (Aor), red/amber (Aam), red/chartreuse (Achar), red/lime (Ali), red/basil (Abas), etc., to various mixtures involving black (Ed, Range, Mber, Hartreuse, Ime, Asil, etc.)—that cannot be rendered on a conventional display. This disparity underscores the motivation for developing an RVGB ("V" for vermillion) screen, whose subpixels are tuned to the primaries of true‐red tetrachromacy. To a true-red tetrachromacy, an RGB screen is missing most of their tetrachromatic hues.

It is important to note that while there are as many perceptually distinct hues between red, yellow, and green in true‐red tetrachromacy as there are colors (with different hue and saturation levels) in normal trichromacy, the available stimuli in everyday life are often “polluted” by overlapping red, yellow and green contributions. When looking at Fig. 11/2 again, we can clearly see why a true-red tetrachromat's "pure yellow" is such a rare color in nature, let alone in human design. To get a perceptually "pure" yellow in true-red tetrachromacy, you need a yellow that's neither polluted by true-red nor by green light. For example, most flowers, paints, and architectural facades that appear yellow are, in fact, broad mixtures of red, yellow, and green light—rendering them as red-yellow when viewed through the true-red tetrachromatic system. To achieve a perceptually pure yellow in true‐red tetrachromacy, one requires a narrow spectral output (approximately 575–595 nm) that is free from extraneous red or green contamination. A light that only emits or a material color that only reflects this narrow wavelength range is highly improbable to find in nature, and basically pointless in human design because normal trichromats will see the many red-yellow-green mixtures as the same metameric lime, yellow, orange, etc. anyway.

In summary, the subjective experience of true‐red tetrachromatic colors is one of dramatic transformation: common stimuli such as RGB S/M/L+) “white” or conventional “yellow” are reinterpreted into entirely new hues that transcend the limitations of trichromacy. This complex interplay between dichoptic spectral filtering and cortical integration not only expands the perceptual color space but also challenges our conventional understanding of color qualia.

Once I find a method to better capture true-red tetrachromatic colors of real life objects with RGB cameras, then I'll provde more examples.

Tetrachromacy is interpreted in various ways across online media, but these interpretations often fall into three general categories.

Statement 1 posits that tetrachromacy does not allow one to see “new” colors because it does not extend vision beyond the normal human visible spectrum. As an example, the YouTuber Knowing Better (2017) claims in his video “Tetrachromats Don’t Have Superpowers” that tetrachromats do not truly see additional colors, only a slightly finer discrimination among similar ones. However, this view overlooks functional tetrachromacy’s fourth color dimension, which indeed provides access to many more color experiences—even if the fourth cone type resides between M and L cone types and thus remains within the standard visible range. 

Statement 2 suggests that tetrachromacy merely lets you see more “intermediate” colors or hues. At one point, I myself believed and helped spread this misunderstanding. Subsequent study and personal experience with true-red non-retinal tetrachromacy (TRNRT) taught me that an extra cone class creates an entirely new dimension of color, yielding not just more (contrasty) intermediate hues but also a two-dimensional hue plane foreign to trichromacy. 

Statement 3 claims that tetrachromacy enables one to see “100 million” colors instead of the typical “1 million,” based on counting around 100 intensity steps per cone type. Although this number can serve as a rough estimate for a fully functional four-cone system, it does not capture the variability in “weakly,” “moderately,” and “strongly” functional tetrachromacy. Even with a supposedly strongly functional retinal tetrachromacy—such as a person with an M' or L' cone sandwiched between the normal M and L peaks—the spectral overlap can reduce its practical impact. If, by contrast, TRNRT’s L+ cone were more distinct from L- than M' (or L') is from M/L, it could arguably be considered more functional in certain respects.

Ultimately, depending on how pronounced the fourth cone’s spectral separation is and how well the observer’s brain integrates that extra input, color vision dimensionality and the number of distinguishable colors may vary substantially. As a result, some people deny that tetrachromacy confers truly new colors, while others concede an improved color discrimination but still think of it as a mere “enhanced trichromacy.” Meanwhile, researchers studying tetrachromacy generally agree that a fully functional additional cone type can scale color vision in a way analogous to how trichromacy differs from dichromacy. In the context of functional tetrachromacy, the first two statements are incorrect, and the third one demands careful nuance.

According to Derval (2015), if an individual can distinguish more than 39 colors in Figure 13.2/1, it is highly probable they possess a functional 4th cone class in the retina. This additional cone enhances contrast perception, an effect that's also noticeable on RGB screens, where tetrachromats are believed to experience a sharper contrast effect. However, despite this correct heightened contrast expected from tetrachromats when looking at the colors of an RGB display, the fundamental limitation remains: an RGB display, with its three subpixels, represents only a 1-dimensional slice of the 2-dimensional tetrachromatic hue plane, and only a 3-dimensional volume of a greater 4-dimensional hypervolume. In essence, although tetrachromats will perceive some colors more vividly or contrasty on such trichromatic displays, they do not actually see a greater number of distinct colors on them than trichromats do. Only contrast is heightened. Testing for tetrachromacy by comparing subjective contrasts of the colors on the 1-dimensional hue line on RGB screens is an inadequate method to test for tetrachromacy by itself because it entirely disregards the 2-dimensionality of tetrachromatic hues, which should be the main method for conducting adequate tests for tetrachromacy.

All online tests claiming to identify tetrachromacy are fundamentally flawed. These flawed tests often rely on images that cannot present a 2-dimensional plane of tetrachromatic hues and often do not even display a continuous gradient; instead, they display trichromatic hues that are not only discretely arranged but sometimes even repeated. A normal trichromat can easily distinguish more than 39 distinct though sometimes similar hues from such an image as seen in Figure 13.2/1, meaning that any "online test for tetrachromacy" does little more than mislead participants into believing they have an enhanced color vision capability. For a display to accurately simulate tetrachromatic vision, it would need a fourth subpixel—in the case of dichoptic true-red tetrachromacy a normal blue and green subpixel, but also a narrow-band vermillion and a deep red subpixel instead of a single reddish subpixel—coupled with appropriate software and color data. Without these modifications, it is impossible to design a direct and reliable test for retinal tetrachromacy on an RGB screen.

An additional layer of complexity arises from the inherent subjectivity of color itself. Wavelengths are objective measurements, but color is the brain’s interpretation of these wavelengths and their multiplex combinations—a qualitative experience known as color qualia. Even though a tetrachromat may make more distinctions between colors compared to a trichromat, this does not necessarily mean they perceive more trichromatic and "more real" colors. Rather, at least concerning the most common phenotype found in human tetrachromats, they are more adept at differentiating variations (i.e. single wavelengths and combinations of multiple narrower wavelength ranges) within the same visible light range that everyone else experiences (at least in M'/L' tetrachromacy). For a comparison between trichromacy and tetrachromacy this means that tetrachromats see many more non-spectral colors, while their color discriminability of natural spectra, as found in rainbows for example, only slightly diverges form the trichromatic norm.

Ideally, testing for tetrachromacy should be conducted by observers who themselves have the genetic predisposition for tetrachromatic vision. However, because functional retinal tetrachromacy is exceptionally rare, the most practical tests are designed by trichromats with in-depth knowledge of the phenomenon and personal experience of simulated tetrachromatic colors. I argue that the current best simulation available is dichoptic true-red tetrachromacy (TRNRT), which allows proficient observers to qualitatively and subjectively understand tetrachromacy not just in theory, but also in practice. Such evaluations rely not only on measurable differences but also on the nuanced, experiential and subjective understanding of color that comes from direct experience with a simulated tetrachromacy.

In summary: while tetrachromacy may enable individuals to discern differences in color that others can't distinguish, the limitations of current RGB-based testing render all online assessments unreliable. The interplay between objective wavelength measurements and subjective color perception further complicates matters, leading to widespread misconceptions about what tetrachromacy truly entails. It is only through carefully designed tests and expert qualitative and behavioral evaluations that the true nature of tetrachromatic vision can be discerned, accurately described and identified.

A hue ordering test for tetrachromacy should be designed with consideration for the 2-dimensionality of tetrachromatic hues. While you can test for tetrachromacy with a (Neitz) anomaloscope in combination with a color vision deficiency test or with a single, well-chosen spectrum of material or light colors [e.g. Citrine (true-red/green) to Saline (black/yellow)] along a confusion line of normal trichromats in respect to the tested tetrachromacy, a complete test for tetrachromacy involves hues that can be arranged in a 2-dimensional structure, or at least a method that considers the 2-dimensionality of tetrachromatic hues.

The below planar and spherical dichoptic true-red tetrachromacy hue ordering tests illustrate complete tetrachromatic hue ordering tests for dichoptic true-red tetrachromacy intended for stereo viewing techniques. With well-chosen material or light colors you can also build analog versions of these tests. However, an analog test is naturally more difficult to create, especially with material colors.

A normal trichromat, as exemplified by the "Trichromatic View" of some of the following programs, will not be able to reasonably and reliably order tetrachromatic hues. Many tetrachromatic hues that appear as distinct hues to tetrachromats will look very similar or even identical and repeating to a normal trichromat. For example, in the "Trichromatic View" there are several identical yellowish colors. Even if a normal trichromat tries to cheat this test by ordering the colors in a subjectively reasonable manner, they would make many mistakes along their lines of confusion and only get some of the tetrachromatic hues correctly ordered by pure chance. This is analogous to a dichromat trying to solve a 1-dimensional, circular hue ordering test for trichromacy.

In the light of this, the aforementioned fake tetrachromacy test by Derval (2015) seems even more erroneous. Not only can you not test for tetrachromacy on a trichromatic (RGB) display, but a 1-dimensional spectrum of hues, especially the way normal trichromatic hues are arranged on an RGB screen, is not nearly enough to test for the full functionality of any tetrachromacy.

At a minimum, a dichromat needs a 0-dimensional, a trichromat a 1-dimensional, a tetrachromat a 2-dimensional, a pentachromat a 3-dimensional, and a hexachromat a 4-dimensional hue ordering test.

Program 13.3/2: Labeled Dichoptic True-Red Tetrachromatic Hue Sphere & Spherical Hue Ordering Test constitutes the same test as Program 13:3/1: Dichoptic True-Red Tetrachromacy Planar Hue Ordering Test, but with the dots arranged on the surface of a sphere. This removes the need for an extra ring of repetitively colored dots, which technically all represent the same distorted singular hue. This spherical structure also makes it easy to find the opposite hue of each dichoptic true-red tetrachromatic hue, because it lies on the opposite side of the hue sphere.

Inspired by Ishihara test plates for diagnosing color vision deficiency Program 13.3/3: Lachner Test For Dichoptic True-Red Tetrachromacy with Ishihara Plates constitutes a higher-dimensional model of the Ishihara test for dichoptic true-red tetrachromacy. This digital test uses stereo viewing techniques that mimic real life dichoptic true-red tetrachromatic colors. If you can distinguish these dichoptic colors and correctly identify the numbers, you can almost identically distinguish them in a similar real-life analog test with the use of the dichoptic true-red tetrachromacy glasses.

(This article is still under construction. To be continued.)