Rainbow Colour Test: Can Counting Shades Reveal Tetrachromacy?

No, counting the stripes in a rainbow chart on your phone cannot tell you how many kinds of colour receptor you have. The image below spread widely in 2015 with a scoring key that sorted viewers into “dichromats”, “trichromats” and “tetrachromats”, and claimed that a quarter of people belonged in each of the outer groups. The numbers were wrong, the animal comparisons were partly wrong, and the method itself cannot work on an ordinary screen.

The chart is still a pleasant thing to look at, and it opens the door to a genuinely interesting subject: how human colour vision works, why some people see fewer colours than others, and what real scientists have found about the rare women who may see more.

What the viral rainbow test claimed

The chart is a horizontal band of 39 colour steps running from violet through blue, green, yellow and orange to red. Readers were asked to count how many distinct shades they could tell apart. The key that came with it went roughly like this:

• Fewer than 20 shades: you supposedly have only two working cone types, “like dogs”, and the post put this group at 25% of people.
• 20 to 32 shades: ordinary three-cone vision, said to cover half the population.
• 33 to 39 shades: a fourth cone type “like bees”, again said to be 25% of people, with the added joke that such viewers dislike yellow clothes.
• More than 39: a reminder that the chart only contains 39 bands, so anyone counting more is miscounting.

It also suggested that people with four cone types would never have been fooled by the famous blue-and-black (or white-and-gold) dress photo from the same year. None of these claims came with a study behind them.

How colour vision really works

The back of the eye is lined with light-sensitive cells. Rods handle dim light and give us little colour information. Cones work in brighter light, and most people have three kinds, each most sensitive to a different band of wavelengths: short (bluish), medium (greenish) and long (yellowish-red). The brain compares the signals from the three types, and that comparison is what we experience as hue. This three-channel system is called trichromacy, and estimates of how many colours a typical person can distinguish run into the millions; Wikipedia’s summary of the research puts the upper estimate at around ten million.

Because colour is built from comparisons, what you see depends on the light reaching your eye, not just on labels like “red” or “blue”. Two surfaces that look identical under one lamp can look different under another. That matters for any test that relies on a printed or glowing image.

Why a screen cannot test for a fourth cone

Every colour on a phone, laptop or television is mixed from three tiny light sources: red, green and blue. The display is designed around normal three-cone vision, so it only produces the combinations a trichromat needs. If a person had a fourth cone tuned somewhere between the usual green and red receptors, the extra information that cone could pick up is simply not present in the light coming off the screen.

That was the point made by researchers at Newcastle University’s tetrachromacy project when Snopes asked them about the viral test in 2015. They said that “computer screens do not provide enough colour information” for such a test. On top of that, the number of bands you count depends on brightness, contrast, night-mode filters, the angle you hold your phone at and how patient you are. Two people with identical eyes can easily give different answers, and the same person can give different answers on two devices.

So a high count does not mean you are a tetrachromat, and a low count does not mean you are colour blind. It mostly measures your screen and your attention.

How common colour blindness really is

The claim that a quarter of people are dichromats is far too high. According to NIH News in Health, about 1 in 76 Americans has some form of colour vision deficiency, and men are affected much more often than women. Most cases are inherited and involve the cones that respond to red and green light, which is why the condition is so often called “red-green colour blindness”.

True dichromacy, where one cone type is missing entirely, is rarer still. Protanopia (no long-wave cones) and deuteranopia (no medium-wave cones) each affect around 1% of men, and tritanopia, a blue-yellow form, is estimated at about 1 in 100,000 people. Many more people have a milder version in which all three cones are present but one is shifted, so certain colours look closer together than they do to others.

The dog comparison is broadly fair: placental mammals as a group are usually dichromats, and dogs fit that pattern. The bee comparison is not. Honeybees are trichromats too, but their three receptors are tuned to ultraviolet, blue and green rather than to the colours humans see. Birds, many fish and some reptiles are the classic examples of four-cone animals.

Tetrachromacy: real, but rare and hard to prove

Human tetrachromacy is not a myth, but it is nothing like 25% of the population. The genes for the red and green photopigments sit on the X chromosome. A woman has two X chromosomes, so if one carries a slightly different version of a cone gene, her retina can end up with four types of cone instead of three. Estimates of how many women carry such a fourth cone vary widely, from about 15% in one study to as many as half in another.

Having the extra cone is not the same as using it. To see more colours, the brain has to treat the fourth signal as a separate channel. Snopes reported that vision researcher Jay Neitz of the Medical College of Wisconsin estimated only around 2% of women might be functional tetrachromats. Neuroscientist Gabriele Jordan of Newcastle University, who has studied the question for decades, told the site she had confirmed true tetrachromacy in a single person over twenty years of testing. That woman, known in the 2010 research as subject cDa29, could tell apart mixtures of light that look identical to people with normal vision, and she was tested with carefully controlled laboratory equipment, not a JPEG.

Men are very unlikely to be tetrachromats because they have only one X chromosome. Interestingly, the mothers and daughters of colour-blind men are among the people researchers look at first, because they are more likely to carry an unusual cone gene.

Common myths about seeing colour

• “Colour-blind people see in black and white.” Almost never. Most see plenty of colour but confuse certain pairs, such as red with green or blue with purple. Complete absence of colour vision is very rare.
• “More shades on a chart means better eyes.” A count on a screen reflects the display, the lighting and your patience more than your retina.
• “Tetrachromats see impossible new colours everywhere.” The confirmed cases show finer distinctions between similar shades, especially in the yellow-green-red range, rather than a different world.
• “The dress photo proves who has four cones.” The blue-and-black versus white-and-gold split is usually explained by how each brain guesses the lighting in the photo, not by cone types.

How to check your colour vision properly

If you enjoy the rainbow chart, enjoy it as a curiosity. If you have a real reason to wonder about your colour vision, perhaps you mix up traffic-light colours, struggle with coloured charts at work, or have a colour-blind relative, the useful step is to talk to an optometrist or doctor. They use standardised plate tests under controlled lighting and, where needed, specialised instruments. Children are often screened at school or at routine eye checks, and an early answer helps with schoolwork that relies on colour coding.

For most people the good news is simple: if you can tell the bands apart well enough to enjoy the picture, your three cones are doing the job they evolved to do.

Sources: Snopes: Do one in four people have a fourth retinal cone?; NIH News in Health: Can’t see certain colors?; Wikipedia: Tetrachromacy; Wikipedia: Dichromacy; Wikipedia: Trichromacy. The rainbow chart circulated via Truth Inside Of You.

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