Every so often a picture goes round with a confident caption. It shows a smooth band of color split into segments, and the text says: count how many distinct colors you can see. Fewer than 20 and you are a dichromat. Around 33 and you are a normal trichromat. More than 39 and congratulations, you are a rare tetrachromat who sees the world in colors the rest of us cannot imagine. It spreads because it flatters, it is quick, and the science word in the middle sounds legitimate.
The test is junk. Not because tetrachromacy is fake, it is real and genuinely fascinating, but because no image on a normal screen can diagnose it. The reason is the same fact that makes your phone able to show a photo at all, and once you see it you will never trust one of those quizzes again.
What a tetrachromat actually is
Most people are trichromats. We have three kinds of cone cell in the retina, each tuned to a different band of the spectrum, roughly long (red), medium (green), and short (blue) wavelengths. Every color you have ever seen is your brain reading the ratio of how hard those three cone types are firing. Three numbers, mixed, give the whole of human color. Estimates put the number of colors a trichromat can tell apart at around a million, a figure the how many colors can you see piece works through in detail.
A tetrachromat has a fourth functioning cone type, usually one that sits between the red and green cones and picks up a slice of the spectrum the standard three handle more coarsely. In theory that extra channel multiplies the combinations enormously. The often-quoted headline is that a true tetrachromat could distinguish on the order of a hundred million colors, a hundred times the trichromat range. The number is a back-of-envelope estimate rather than a measured fact, but the direction is real: a fourth independent input means finer slices of color the rest of us blend into one.
The genetics line up with this being almost entirely a female trait. The genes for the red and green cones sit on the X chromosome. A woman carries two X chromosomes, so if one carries a slightly shifted version of a cone gene, she can end up expressing four distinct cone types instead of three. Men, with a single X, do not get the same second copy. Gabriele Jordan, who has spent years on this at Newcastle University, estimates that perhaps 12 percent of women carry the genetic setup for a fourth cone (Discover Magazine).
Carrying four cones is not the same as using them
Here is the part the viral quizzes skip. Having a fourth cone in your retina does not mean your brain does anything useful with it. The wiring that turns cone signals into the experience of color has to learn to treat that fourth input as a separate channel, and in most carriers it apparently does not. They have the hardware and run it as an ordinary trichromat anyway.
Jordan and John Mollon put real numbers on this in a 2010 study. They tested 24 women who genetically carried a fourth cone. Almost all of them performed like normal trichromats on a task built specifically to expose a fourth channel. One subject, recorded in the paper as cDa29, was different. She picked the odd stimulus out every single time, scoring in a way a three-cone visual system simply cannot (Journal of Vision, Jordan et al. 2010). She is the clearest confirmed case in the literature of a functional tetrachromat: someone whose brain genuinely uses the extra cone. The painter Concetta Antico is the best-known public example, and her canvases are full of color distinctions she insists are simply there in front of her.
So the honest population picture is layered. Many women carry the gene. Far fewer turn it into a fourth working channel. The number of rigorously confirmed functional tetrachromats you can point to is tiny. That rarity is exactly why a one-tap image quiz claiming to find them by the thousand should make you suspicious.
Why a screen cannot test for it, full stop
Now the core problem, and it is not a matter of the quiz being sloppy. It is physics. Your screen builds every color out of exactly three primaries: red, green, and blue subpixels, brightened and dimmed in combination. That is the whole toolkit. When you see yellow on a display there is no yellow light leaving it, only red and green emitters firing together while your red and green cones add up to the sensation of yellow. The what color do two colors make article walks through this additive trick in full.
Three primaries can only ever stimulate the three standard cone types in fixed, locked-together ratios. The one thing a fourth cone needs in order to reveal itself is a color that lights it up independently of the other three, a stimulus where the fourth channel says one thing while red, green, and blue say another. A red-green-blue screen cannot produce that stimulus. It has no fourth primary to pull the fourth cone away from the pack. So to a tetrachromat and a trichromat looking at the same monitor, the available colors collapse onto the same three-cone description. The extra cone has nothing to grab onto.
Put bluntly, you cannot be tested for tetrachromacy on the device you are reading this on, and neither can anyone else. Counting the segments in a gradient strip measures something, but that something is your monitor's bit depth, its calibration, the ambient light in your room, and how patiently you stare. A cheap panel banding a gradient into visible steps will hand you a higher count than a good one showing the same gradient smoothly. The quiz rewards a worse screen. That is the opposite of a vision test.
How tetrachromacy is really tested
Real testing throws out the screen and controls the light directly. The classic instrument is a variation on the Rayleigh match, long used to diagnose color blindness. A subject looks into an eyepiece at a split field and adjusts a mixture of red and green light until it matches a reference yellow. Trichromats settle on a fairly wide range of mixtures that all look like an acceptable match. A fourth cone narrows that range, because the extra channel notices differences between mixtures that look identical to three cones. The pattern of which matches a person accepts and rejects is the tell.
Jordan's confirming test used the same idea in a forced choice. In a dark room the subject saw circles of light, one of which was a precise red-green mixture computed to look identical to the others for a trichromat but distinct for a tetrachromat. Pick the odd one out, round after round. A trichromat scores at chance because to her eyes there is no odd one. cDa29 scored perfectly. The whole apparatus exists to do the one thing a consumer screen cannot: generate light that addresses the fourth cone on its own terms.
Other labs use finely dyed physical samples, two cloths or chips that reflect genuinely different spectra yet look the same to standard vision. A tetrachromat can sort them apart. The common thread is real light with real spectral content, never a three-primary emulation of it. If a test runs in your browser, it is not this.
What an online color game can honestly tell you
None of this means color tests on a screen are worthless. They just measure a different thing, and it is worth being clear about which. A screen cannot count your cones. It can absolutely measure your color discrimination: how small a difference between two colors you can reliably notice. That is a real, trainable skill, and it varies a lot between people who all have exactly three cones.
That is the honest claim behind the games on this site. When you guess a color from memory or chase a target shade with the sliders, the scoring runs on CIEDE2000, the industry formula for how far apart two colors look to a normal human eye. A high score means you place colors close to a target that most people would miss by a wider margin. It does not mean you have a fourth cone. It means your three are well trained and your visual memory is sharp, which is the thing designers, photographers, and painters actually develop on the job.
If you want the closest thing to a legitimate color-vision screen you can run at home, the Ishihara-style plates in the game are the honest version. They are built to flag red-green color blindness, the common and well-understood end of cone variation, and unlike a tetrachromat quiz they are testing for a deficit that a three-primary screen genuinely can show. They will not crown you a tetrachromat. Nothing on a screen can. But they will tell you something true about how your cones are wired, which is more than the viral strip has ever done.
So the next time the color-counting quiz comes round, you can answer it in one line. Whatever number you got, it is your monitor's score, not your retina's. Real tetrachromacy is rarer, stranger, and quieter than a shareable image lets on, and the only honest way to find it is to leave the screen behind entirely.