Most disagreements about color can be blamed on poor lighting, personal taste, or the fact that paint manufacturers have somehow created 700 varieties of beige and given each one an emotionally significant name.

Occasionally, however, two people may look at the same objects and genuinely see different colors—not because one of them is mistaken, but because one of them may possess visual equipment the other person does not have.

Most humans have three types of color-detecting cone cells. A small number of people may have four. An even smaller number appear capable of using that fourth cone to distinguish colors the rest of us cannot perceive.

This condition is called tetrachromacy, and it raises a tantalizing question: Are some people walking around in a world containing colors the rest of us cannot even imagine?

This is a particularly intriguing possibility for this writer, who is colorblind but nevertheless previously undertook the heroic tasks of explaining how to see impossible colors and clarifying what colorblindness is—and is not. We are now forced to consider that some people may not only see colors we cannot identify; they may see distinctions our eyes have never bothered to submit to the brain for consideration.

Can some people see colors others cannot? That is the surprisingly colorful question that leads us into an attempt to provide a black-and-white answer.

How Three Cones Create the Colors You See

Color does not exist in light in quite the way we tend to imagine it. A beam of light does not arrive carrying a tiny label that says, “Hello. I am yellow.”

Light consists of electromagnetic radiation at different wavelengths. The retina detects that light and sends signals to the brain, which turns the information into the experience we call color. Color is therefore not simply a property of the outside world. It is the brain’s interpretation of what the eyes report.

The retina contains two major kinds of light-sensitive cells. Rods are particularly useful in dim conditions, although they are not much help in distinguishing one color from another. Cones operate best in brighter light and provide color vision.

People with typical color vision have three kinds of cones:

• S-cones, most sensitive to shorter wavelengths;
• M-cones, most sensitive to medium wavelengths; and
• L-cones, most sensitive to longer wavelengths.

These are commonly called blue, green, and red cones, although that shorthand is slightly misleading. Each cone responds to a broad and overlapping range of wavelengths. Your brain determines color primarily by comparing how strongly the different cone types respond.

Yellow, for example, does not require a special yellow cone. It emerges from a particular pattern of activity in the L- and M-cones. Purple results from another combination. Magenta is especially interesting because there is no single wavelength of magenta light. The brain manufactures the experience when the cone signals present it with a situation for which the visible spectrum has neglected to provide a convenient answer.

This three-cone arrangement makes most humans trichromats. It is also why televisions, cameras, and computer displays rely on combinations of red, green, and blue. Three carefully selected primary colors can produce enough combinations to persuade the ordinary human visual system that it is looking at a much larger assortment.

Color Perception: Different Light, Same Color

Trichromatic vision is remarkably effective. It gives us sunsets, autumn leaves, warning labels, and the ability to spend forty-five minutes deciding between two shades of paint that will look identical once they are on the wall. It is not, however, especially sentimental about preserving information.

Two objects can reflect entirely different mixtures of wavelengths yet trigger the same pattern of activity in your three cone types. When that happens, they look like the same color even though the light arriving from them is physically different. Scientists call these matching colors metamers, because “two completely different things your eyes have carelessly filed under the same name” was apparently considered too long.

Suppose one object reflects a narrow band of genuinely yellow light. Another reflects a carefully balanced mixture of red and green wavelengths. Their spectral fingerprints are different, but if both combinations stimulate your three cone types in the same proportions, your brain shrugs, stamps both files “yellow,” and moves on to more important matters.

A person with a fourth independently functioning cone might not be so easily fooled. That extra cone could respond differently to the two mixtures, revealing that the supposedly identical yellows are not identical at all. What looks like one color to the rest of us might split into two plainly distinct colors for a tetrachromat.

That is the real promise of tetrachromacy. It does not necessarily mean seeing ultraviolet light, infrared radiation, or the unsettling aura surrounding your neighbor’s cat. It means distinguishing between colors that ordinary human eyes confidently—and incorrectly—declare to be the same.

Where Does a Fourth Cone Come From?

The genes that produce the pigments used by the L- and M-cones are located on the X chromosome. Most men have one X chromosome and therefore one set of these particular instructions. Most women have two X chromosomes and can inherit different versions of a cone-pigment gene from each parent.

Having two X chromosomes does not mean every retinal cell uses both copies. Early in development, each cell largely switches off one X chromosome through a process called X-inactivation. The choice is random, so one group of retinal cells may use the cone-pigment gene from one X chromosome while another group uses the version on the other X.

If the two versions produce pigments with meaningfully different sensitivities, the retina may end up containing four cone classes rather than three: the ordinary S- and M-cones, for example, plus two slightly different varieties of L-cone.

This particular genetic lottery is therefore played mainly on two X chromosomes, which is why researchers looking for human tetrachromats have concentrated primarily on women—especially women whose sons, fathers, or other maternal male relatives have a mild form of red-green color deficiency.

The family connection produces one of biology’s more peculiar arrangements. A man may have difficulty distinguishing certain reds and greens because he has an altered cone pigment. His daughter may inherit that pigment along with a different version on her other X chromosome and potentially end up with more varieties of cone than the average person.

Nature does not merely give with one hand and take with the other. Sometimes it takes something from one family member and quietly hands the upgrade to someone else.

Four Cone Types Are Not Enough

Possessing four cone pigments does not automatically mean someone experiences four-dimensional color vision.

The extra pigment may be so similar to one of the standard pigments that it adds very little useful information. More importantly, the signals from all four cone types must remain sufficiently separate as they travel through the retina and brain. The visual system must compare and interpret the fourth signal rather than quietly combining it with one of the original three.

Researchers sometimes distinguish between retinal or weak tetrachromacy, in which four cone classes are present, and functional or strong tetrachromacy, in which the observer can actually use the fourth signal to make color distinctions unavailable to ordinary trichromats.

This distinction is responsible for much of the confusion surrounding claims about how many tetrachromats exist.

You may encounter statements declaring that 12 percent of women are tetrachromats or that tetrachromats can see 100 million colors. The first figure appears to have escaped from research estimating how many women carry genes associated with mild anomalous trichromacy—which is not the same thing as demonstrating functional tetrachromacy. Both claims have nevertheless been repeated often enough to acquire the polished appearance of established fact, thereby inspiring one of humanity’s more perilous conclusions: “I read it on the internet, so it must be true.”

Genetic variations capable of producing four retinal pigments may be relatively common, depending on which variations are counted. Functional tetrachromacy appears to be much rarer. There is not enough evidence to assign a reliable percentage to the number of people who actually possess and use an independent fourth color channel.

Nor is there a scientifically tidy way to count every distinguishable color available to a human observer. The answer changes with lighting, brightness, surrounding colors, viewing conditions, and the standard used to decide when two nearly identical shades qualify as separate colors. “One hundred million colors” is less a measurement than an extremely enthusiastic brochure.

The Woman Known as cDa29

The strongest experimental evidence for functional human tetrachromacy comes from a woman identified in scientific literature as cDa29, a designation that makes her sound less like a research participant and more like a droid assigned to monitor paint samples on a Rebel cruiser.

In a 2010 study published in the Journal of Vision, researchers Gabriele Jordan, Samir Deeb, Jenny Bosten, and John Mollon tested women who carried genes associated with anomalous red-green color vision.

One of the experiments involved showing participants three successive lights. Two were monochromatic yellow. The third was a mixture of red and green light that could be adjusted until it appeared identical to the yellow lights.

For a typical observer, there is some combination at which the mixture and the monochromatic yellow become indistinguishable. In that part of the spectrum, the S-cones contribute little, leaving the observer dependent mainly on the responses of the M- and L-cones. Adjust the mixture until those two responses match, and the ordinary color perception system has no remaining witness willing to testify that anything is amiss.

A strong tetrachromat should have an additional cone response that prevents the mixture from perfectly matching the monochromatic yellow. No matter how the researchers adjust the red-green proportions, the fourth cone should continue noticing that the lights are physically different.

That is essentially what happened with cDa29. She repeatedly identified the mixed light under conditions that left the control participants insisting, with complete sincerity, that there was nothing to identify. Genetic analysis suggested that she possessed three well-separated cone pigments in the long-wavelength portion of the spectrum, in addition to the ordinary short-wavelength pigment. In other words, where the rest of the group saw one yellow and wondered why the scientists kept asking, cDa29 apparently saw the visual equivalent of two suspects wearing the same coat.

Of the 24 obligate carriers of deuteranomaly examined in the study, she was the only one who behaved like a tetrachromat across all the researchers’ tests. The others may have possessed some of the necessary genetic equipment, but their visual systems did not appear to be doing anything especially useful with it. Biology, as usual, had supplied the parts without guaranteeing that anyone had read the assembly instructions.

The result does not tell us how many functional tetrachromats exist in the general population. These women had already been selected because their genetics made them unusually promising candidates, so the study was less a random census than an audition composed entirely of people who had already made the callback list. It does show, however, that carrying the relevant genes is usually not enough—and that at least one person performed exactly as a genuine functional tetrachromat should.

What Does a Tetrachromat Actually See?

This is the point at which science produces an answer that is both fascinating and deeply unsatisfying: We do not know.

Researchers can determine whether someone distinguishes stimuli that look identical to ordinary observers. They can measure cone pigments, manipulate wavelengths, and record correct responses. They cannot climb inside another person’s conscious experience and inspect what the resulting color looks like.

A human tetrachromat probably does not see an entirely different visible spectrum. The extra cone associated with the best documented cases appears to be positioned between the usual M- and L-cone sensitivities. It does not ordinarily reach into ultraviolet or infrared territory.

The difference would most likely be especially useful among colors involving reds, greens, yellows, and the many intermediate mixtures that the rest of us compress into the same categories.

Imagine two fabric samples, flower petals, or paint chips that look perfectly identical to you. A tetrachromat might see an obvious distinction between them. To her, insisting that the colors match could sound like someone claiming a trumpet and a violin sound identical because both instruments were playing the same song.

Whether she experiences wholly novel hues or simply much finer divisions among familiar ones is harder to answer. A fourth independent receptor does not merely add more points to the same three-dimensional color space. In principle, it adds another dimension.

Unfortunately, explaining a fourth color dimension to a trichromat is rather like explaining red to someone who has never seen it. We lack the sensory reference necessary to imagine the experience. Language can tell us that the distinction exists but cannot deliver the perception itself.

Tetrachromacy should not be confused with the color-perception differences discussed in our earlier article, “Did Ancient People See Blue?” Members of the Himba people of Namibia reportedly had difficulty identifying a blue square among green ones, yet could quickly detect subtle differences between shades of green that often left English speakers staring at the screen as though the correct answer might eventually become embarrassed and reveal itself. That was not evidence that the Himba possess different cone cells. Their eyes received essentially the same visual information; their language and culture had simply trained them to divide, label, and notice that information differently.

Tetrachromacy begins one step earlier. A functional tetrachromat may receive additional sensory information from a fourth cone type, allowing her to distinguish colors that produce identical signals in an ordinary trichromatic retina. The Himba example involves a different filing system for documents already delivered to the brain; tetrachromacy may involve receiving an extra document that never reaches everyone else’s desk. One changes which distinctions the brain has learned to emphasize. The other may provide a distinction that three-coned observers are physically incapable of seeing.

Why a Computer Cannot Show You Tetrachromatic Colors

This limitation also means no website can simply display a square labeled “the color only tetrachromats see.”

Your screen produces colors with red, green, and blue primaries. Those three adjustable channels can generate only a three-dimensional range of cone responses. A tetrachromat may respond to those primaries differently from an ordinary observer, but an RGB display cannot independently control four cone classes or reproduce every distinction available in a four-dimensional color space.

A conventional camera creates a similar bottleneck by reducing the spectrum reaching each pixel to three channel values. Because camera sensors do not perfectly duplicate human cone sensitivities, they may preserve some distinctions and lose others. What they cannot do is record the complete spectrum or guarantee preservation of every distinction available to a tetrachromat. By the time the resulting RGB values reach an ordinary three-primary display, any information requiring an independently controlled fourth channel has failed to receive an invitation.

This is why online images asking, “How many color bands can you see?” cannot diagnose tetrachromacy. The number of bands depends on the image compression, screen calibration, brightness, ambient lighting, and whether someone has become determined to count divisions that are mostly visible because the graphic was badly made.

Newcastle University’s Tetrachromacy Project states plainly that ordinary computer screens do not provide enough color information to test for the condition. A legitimate diagnosis requires carefully controlled physical stimuli, not a gradient circulating on social media between a personality quiz and a photograph of a suspiciously large house cat.

Would the Brain Know What to Do With Another Cone?

The eye is only the beginning of color vision. The brain must interpret the incoming signals, which raises another question: If an additional cone suddenly appeared, would the brain know how to use it?

A remarkable experiment involving squirrel monkeys suggests the adult visual system may be more adaptable than scientists once assumed.

Male squirrel monkeys ordinarily have two types of color cone and therefore lack the red-green discrimination available to trichromats. In a 2009 gene-therapy experiment, researchers introduced a human L-cone pigment into the retinas of adult monkeys.

After the new pigment became active, the monkeys gained the ability to distinguish colors they previously could not. They did so without having received the new pigment during infancy and without surgeons installing an entirely new bundle of dedicated neural wiring.

The experiment did not create human tetrachromats. It moved monkeys from two functional cone classes to three. Nevertheless, it showed that an adult primate brain can extract useful color information from a newly introduced receptor signal. The brain apparently did not require a committee meeting, a multiyear feasibility study, or even a revised employee handbook before putting the new information to work.

That finding makes functional human tetrachromacy more plausible, but it does not guarantee it. The fourth pigment must still differ sufficiently from the others, its signal must survive retinal processing, and the brain must treat it as independently meaningful. Nature has arranged several consecutive opportunities for the entire project to be quietly canceled.

Tetrachromatic Colors Are Not the Same as Impossible Colors

Tetrachromacy should not be confused with the impossible and chimerical colors discussed in our earlier article.

So-called impossible colors cover more than one visual parlor trick. Chimerical colors can arise through adaptation and afterimages, producing experiences that seem more saturated than ordinary colors, self-luminous, or strangely dark and colorful at the same time. Experiments involving “forbidden” combinations such as reddish green or yellowish blue instead attempt, under carefully controlled conditions, to interfere with the opponent-color system that ordinarily prevents those pairings from appearing together.

A tetrachromat is doing something different. She is not coaxing three cone channels and their opponent pathways into an unusual state. She may be receiving a fourth stream of spectral information that most human eyes never collect.

There is, however, an intriguing point of contact between the two subjects. In 2025, researchers at the University of California, Berkeley, used a system called Oz to stimulate selected retinal cones with extraordinary precision. By activating M-cones without producing the usual accompanying response in L-cones, they caused five participants to perceive an intensely saturated blue-green color they called “olo”.

The participants had not grown a fourth cone. The researchers instead used lasers to produce a pattern of cone activity that natural light cannot ordinarily create. The result provided another reminder that the colors available in everyday life do not necessarily exhaust the experiences the human brain is capable of generating.

Could You Be a Tetrachromat?

Possibly, but being unusually good at naming paint colors is not enough.

The most promising candidates are women whose sons, fathers, maternal uncles, maternal grandfathers, or other close male relatives have very mild red-green color-vision deficiencies. Genetic testing can determine whether someone carries distinct cone-pigment variants, but that establishes only the potential for tetrachromacy.

Functional tetrachromacy requires laboratory tests showing that the person reliably distinguishes colors that ordinary trichromats cannot distinguish after brightness and other clues have been eliminated.

Artists, designers, and people who work extensively with color may become exceptionally skilled at noticing subtle differences through practice. That expertise is impressive, but it is not proof of an additional cone type. A trained trichromat may outperform an inattentive potential tetrachromat who has spent her life using the phrase “sort of brownish” and moving on to more pressing matters.

A genuine tetrachromat may also have no reason to suspect anything unusual. She has seen the world through the same eyes her entire life. Other people may occasionally insist that two colors match when they plainly do not, but there is no obvious reason to conclude from this that one possesses a rare sensory ability rather than that everyone else is careless.

To be fair, history suggests that assuming everyone else is careless will provide the correct answer surprisingly often.

Other Animals Have Been Doing This for Ages

Tetrachromacy may sound like a human superpower, but it is not particularly exotic in the animal kingdom. Many birds, fish, reptiles, and insects possess four or more kinds of photoreceptors.

In many birds, the fourth cone is sensitive to ultraviolet wavelengths. Plumage that looks plain to us may contain brilliant ultraviolet patterns used for courtship, identification, or signaling. Flowers also display ultraviolet markings that guide pollinating insects toward nectar.

The natural world is therefore filled with visual information aimed at audiences that do not include us. Humanity has spent centuries congratulating itself on being the pinnacle of creation while standing in the middle of a spectacular light show it lacks the equipment to notice.

The Hidden Colors Around Us

So, can some people see colors others cannot?

The most defensible answer is yes—with several important qualifications.

Some people almost certainly possess four distinct cone pigments. Merely having the extra cone does not establish functional tetrachromacy. Strong experimental evidence remains rare, although at least one carefully studied participant has demonstrated color discrimination consistent with a genuine fourth visual channel.

We cannot know precisely what that experience looks like. We cannot faithfully reproduce it on a normal screen, capture its full four-dimensional character with a conventional RGB photograph, or describe it using color words developed by people who share only three cone classes.

That may be the most fascinating part of tetrachromacy. It reminds us that what we experience as the complete and obvious appearance of reality is only one biological interpretation of the available information.

The sunset you see may not be the sunset someone else sees. Two flowers that look identical to you may be visibly different to another observer. The paint samples you carefully matched may, to a tetrachromat, resemble four unrelated colors that happened to arrive at the store in the same trench coat.

There may be colors surrounding us that we will never perceive—not because they are impossibly distant or hidden beyond the visible spectrum, but because our eyes look directly at them and report that there is nothing more to see.

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