Imagine standing beside someone, watching the same sunset. You see the sun as a soft, glowing ember melting into gold as light scatters across the sky. Yet, the person beside you sees subtle distinctions between shades that appear completely identical to you. This is not because they are more observant, but because their eyes may be biologically equipped to detect differences that yours cannot.
This remarkable phenomenon, known as tetrachromacy, challenges a common assumption about human perception: that we all experience the same world through the same colours. Hidden within the retina, an additional class of colour-sensitive cones may allow some individuals to distinguish subtle differences between colours that appear identical to the average person (1).
Tetrachromacy reveals a profound truth: perception is not a direct reflection of reality; it is a biological interpretation of it.
How do most humans see colour?
Before we can understand what it might mean to possess a fourth cone type, we must first consider how most humans construct colour from three cones.
When light enters the eye, it passes through the pupil and is focused onto the retina, a layer of light-sensitive tissue lining the back of the eye. Within the retina are specialised photoreceptor cells: rods and cones. Rods are particularly important for low-light and night vision, while cones allow us to perceive colour and fine detail under bright light.
Most humans possess three cone classes that are most sensitive to short, medium and long wavelengths, commonly abbreviated as S, M and L cones, respectively. A single cone cannot encode colour independently, as each responds to a broad, overlapping range of wavelengths. Colour perception therefore depends on comparing patterns of activity across all three cone classes. These signals are further organised into opponent pathways, including red-green and blue-yellow channels, alongside a luminance or brightness pathway. This allows the nervous system to transform three overlapping photoreceptor signals into the wide range of distinct shades that make up ordinary trichromatic vision (2).
Nevertheless, trichromacy has limitations. Physically different mixtures of light wavelengths can sometimes trigger the exact same pattern of activity across our three cone classes, making them appear identical. These perceptually identical colours are known as metamers.
A functional fourth cone, however, might allow an individual to distinguish between some of these mixtures, revealing visually distinct shades that most people see as identical colours.
Where could a fourth cone come from?
The possibility of human tetrachromacy begins with cone photopigments, the light-sensitive molecules that allow cones to respond to light. Each contains a protein called an opsin, which helps determine the range of wavelengths to which that cone responds. The genes encoding the L and M cone opsins are located close together on the X chromosome and can vary between individuals (3).
Females usually inherit two X chromosomes and may receive different opsin variants from each parent. In every cell, one X chromosome is largely inactivated. However, the same X is not necessarily inactivated in every cell. Therefore, different retinal cones may express different opsin variants depending on which chromosome remains active in each cell.
Consequently, some women — particularly carriers of genetic variants associated with red-green colour-vision deficiencies — may express four spectrally distinct cone photopigments rather than the usual three. Because X-inactivation can produce a mosaic retina, however, the additional photopigment may not be distributed evenly or be present in sufficient numbers to substantially influence vision (3).
However, this does not mean that all women carrying these variants experience tetrachromatic colour, as possessing the necessary receptors does not guarantee that the visual system can use the information they provide.
Do four cones mean four-dimensional colour vision?
Possessing an additional photopigment is not equivalent to gaining an additional dimension of colour perception. Four cone types may still feed into the usual neural pathways, producing only subtle improvements in colour discrimination. Functional tetrachromacy requires the visual system to preserve and compare four distinct sources of information. This would enable the observer to consistently distinguish colours that appear identical to trichromats.
For the additional photopigment to be useful, it must respond differently enough from the existing pigments to provide distinct information. The cone classes must also be expressed in suitable proportions, while their signals must remain distinguishable through subsequent visual processing. Finally, the brain must be capable of interpreting the additional information (1).
This distinction complicates estimates of how common tetrachromacy is. Genetic studies can identify people who may possess four photopigments, but cannot determine whether those signals produce functional tetrachromatic vision. The genetic potential for tetrachromacy may therefore be more common than its functional expression, although the prevalence of functional tetrachromacy remains unknown (1).
Does the brain know what to do with a fourth signal?
Colour perception does not end at the retina: cone signals are reorganised by retinal circuits, transmitted through the optic nerve and processed across multiple regions of the brain before becoming part of conscious experience. In typical vision, cone signals are compared through opponent pathways. Some neural pathways compare the activity of L and M cones, contributing to red-green distinctions. Others compare S cone activity with combined signals from L and M cones, contributing to blue-yellow distinctions (4).
Simply possessing an additional cone type does not automatically create an additional opponent pathway. Its output could be combined with an existing channel instead of being preserved as an independent source of information. If that occurs, a person could possess four cone photopigments while continuing to perceive an essentially trichromatic world.
This is where the distinction between possessing four photopigments and actually seeing tetrachromatically becomes particularly important. Professor Trichur Vidyasagar, a neuroscientist specialising in visual perception at the University of Melbourne, explained this distinction in an interview for OmniSci Magazine. Functional tetrachromacy would require two things: a sufficiently distinct fourth photopigment and neural pathways capable of preserving its signal separately (5). Whether the fourth signal forms an additional opponent channel, modifies an existing one, or becomes distinguishable only at later stages of visual processing remains a central question.
How the brain would respond to an additional signal remains uncertain. In one study, adult squirrel monkeys developed trichromatic behaviour after gene therapy introduced a third opsin (6). This suggests that early exposure may not always be necessary for the brain to use additional colour information. However, whether the human visual system could similarly support four-dimensional colour vision remains unresolved.
Even if that signal is functionally preserved, the result may be less fantastical than popular descriptions suggest. Rather than granting access to an entirely new palette, Vidyasagar suggested that tetrachromacy would more likely allow finer discrimination between shades. “It’s not as if there’s new colours they can see,” he explained (5).
How can scientists test tetrachromacy?
Proving functional tetrachromacy is considerably more difficult than identifying a genetic variant associated with an additional photopigment. Self-reported experiences of unusually vivid or nuanced colour are subjective, while strong performance on ordinary colour-discrimination tests does not necessarily demonstrate an additional dimension of colour vision.
Dr Amanda Douglass is a senior lecturer in the Department of Optometry and Vision Sciences at the University of Melbourne. In an interview for OmniSci Magazine, Douglass distinguished between two questions. Does an individual possess an additional cone class, and does the visual system use its signal functionally? She said convincing evidence would need to demonstrate both an additional cone and a measurable difference in how its signal is processed through neural pathways (7).
Researchers therefore rely on carefully controlled psychophysical experiments. In colour-matching tasks, for example, participants adjust mixtures of primary lights until two stimuli appear identical. Typical trichromatic vision can be described using three independent primaries. In principle, demonstrating a fourth independent dimension would require matches that cannot be explained with this three-primary framework (1).
Experiments must also rule out alternative explanations, including brightness cues, lighting conditions, learning and chance.
Why can an online test not identify a tetrachromat?
Ordinary computer screens present another problem: they generate colours by combining three primaries — red, green and blue. A standard RGB display therefore cannot independently reproduce all of the spectral information that might be required to test a hypothetical fourth cone class. Specialised displays can use additional primaries, but ordinary monitors cannot (8).
Calibration creates an additional limitation. As Douglass asked, “What’s red on your screen? Is that the exact same red as on my screen?” Not necessarily. Differences in calibration and display technology mean that an online colour test cannot guarantee that every participant is viewing the same stimulus (7).
What does the scientific evidence show?
One of the strongest pieces of evidence comes from a study by Jordan and colleagues. The researchers investigated women carrying genetic variants associated with anomalous trichromacy. They combined colour-matching experiments, tests of the participants’ perceptual colour space and molecular genetic analysis to search for evidence of four-dimensional colour vision (9).
For most participants, possessing four cone classes did not translate into functional tetrachromacy. However, one of the 24 obligate carriers of deuteranomaly demonstrated tetrachromatic behaviour across every test. Molecular analysis showed that she possessed three well-separated photopigments sensitive to longer wavelengths, alongside her short-wavelength photopigment (9).
The evidence is persuasive because her tetrachromatic behaviour appeared across several independent tests, rather than within a single colour-matching task. The molecular analysis also supported the behavioural results by confirming that her photopigments could provide four distinct retinal signals.
However, the finding came from only one participant within a small and highly specific sample. The experiments also measured performance under controlled laboratory conditions, not her everyday colour experience or neural processing directly. The study therefore suggests that functional human tetrachromacy is possible, but cannot establish its prevalence or reveal precisely how tetrachromats experience colour (9).
Can tetrachromats really see 100 million colours?
Popular accounts often claim that typical trichromats see approximately one million colours, while tetrachromats see 100 million. These figures are theoretical estimates, not experimentally counted inventories of consciously perceived colours (1).
Douglass was unequivocal when asked whether the 100 million figure should be taken literally: “Very cautiously” (7).
The number of distinguishable colours depends on how researchers define a “different” colour, alongside illumination, testing conditions and the region of colour space examined. Current evidence supports a narrower conclusion: some individuals may be able to distinguish between stimuli that appear identical to typical trichromats. Vidyasagar explained that tetrachromacy would most plausibly provide finer discrimination between subtle colour variations, rather than access to an entirely new palette (5). Science may therefore support enhanced colour discrimination, but it cannot yet attach a reliable number to that ability.
Why does tetrachromacy matter?
Colour perception influences everyday decisions, from interpreting traffic signals to judging whether food is ripe. It also shapes how we communicate information through maps, medical images, screens and warning systems. Understanding differences in colour perception therefore matters even beyond the experience of potential tetrachromats.
Tetrachromacy offers a natural experiment in how genes, sensory receptors and neural circuitry combine to construct perception. Two people can encounter the same spectrum of light while differences in their sensory biology affect the information they can extract from it. Studying this variation exposes the limits of assuming that everyone experiences colour in the same way (10).
This research could also refine how colour vision is assessed. Standard clinical tests primarily identify colour-vision deficiencies. Tetrachromacy raises the opposite question: how can researchers detect visual information beyond the usual trichromatic framework? Although its practical applications remain uncertain, tetrachromacy highlights limitations within tests and technologies designed around typical three-cone vision.
More broadly, tetrachromacy demonstrates that our sensory experience is not identical to the physical world itself. The nervous system samples only part of the available information and constructs a perceptual representation from it. A fourth cone — whether or not the brain can use its signal — reveals how profoundly perception depends on the biology of the perceiver (10).
Conclusion
The possibility of tetrachromacy reminds us that seeing the same world does not necessarily mean experiencing it in exactly the same way. A fourth cone may not unlock an entirely new palette of unimaginable colours. For some individuals, however, it may reveal subtle distinctions that remain indistinguishable to most of us.
As Vidyasagar put it, “What is my green may not be the same green to you” (5). His observation raises a question extending far beyond tetrachromacy. How much of what we perceive belongs to the world itself, and how much is constructed by our biology?
Perhaps, then, two people watching the same sunset really can see it differently. The light reaching their eyes may be the same, but their brains may not construct the same perceptual world.
References
Jordan G, Mollon JD. Tetrachromacy: the mysterious case of extra-ordinary color vision. Curr Opin Behav Sci. 2019;30:130-134. doi:10.1016/j.cobeha.2019.08.002
Gegenfurtner KR. Cortical mechanisms of colour vision. Nat Rev Neurosci. 2003;4(7):563-572. doi:10.1038/nrn1138
Neitz J, Neitz M. The genetics of normal and defective color vision. Vision Res. 2011;51(7):633-651. doi:10.1016/j.visres.2010.12.002
Dacey DM, Packer OS. Colour coding in the primate retina: diverse cell types and cone-specific circuitry. Curr Opin Neurobiol. 2003;13(4):421-427. doi:10.1016/s0959-4388(03)00103-x
Vidyasagar T. Interview by Fernando-Nanayakkara A. Personal interview. September 2, 2026.
Mancuso K, Hauswirth WW, Li Q, et al. Gene therapy for red-green colour blindness in adult primates. Nature. 2009;461(7265):784-787. doi:10.1038/nature08401
Douglass A. Interview by Fernando-Nanayakkara A. Personal interview. September 10, 2026.
Smith VC, Pokorny J. Chromatic-discrimination axes, CRT phosphor spectra, and individual variation in color vision. J Opt Soc Am A Opt Image Sci Vis. 1995;12(1):27-35. doi:10.1364/josaa.12.000027
Jordan G, Deeb SS, Bosten JM, Mollon JD. The dimensionality of color vision in carriers of anomalous trichromacy. J Vis. 2010;10(8):12. Published 2010 Jul 1. doi:10.1167/10.8.12
Bosten JM. Do you see what I See? Diversity in human color perception. Annu Rev Vis Sci. 2022;8:101-133. doi:10.1146/annurev-vision-093020-112820
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