Colour Vision Deficiency, Explained
What colour blindness is: protan, deutan and tritan types, how common it is, how an Ishihara-style color blind test works, and why a screen test only screens.
By Udit Agarwal · Updated October 17, 2026
Colour vision deficiency, usually called colour blindness, is a reduced ability to tell some colours apart, most often reds and greens, because one of the three kinds of cone cell in the eye works differently or is missing. According to MedlinePlus Genetics, from the US National Library of Medicine, the red-green form affects about 1 in 12 males and 1 in 200 females with Northern European ancestry. Most people with it still see colour; they miss some of the differences other people see.
The retina has three kinds of cone cell, each most sensitive to a different part of the spectrum: short wavelengths (S cones, often called blue), medium (M, green) and long (L, red). The colour names are shorthand, since the M and L cones respond to heavily overlapping ranges. The brain works out colour by comparing how strongly the three kinds respond. If one kind responds differently or is missing, some colours that look different to most people send the same pattern of signals and look alike. Vision scientists say such colours lie on a confusion line, and colour vision tests are built from them.
Deficiencies are named after the cone type that is affected, protan for L, deutan for M and tritan for S, and graded by how strongly it is affected:
| Type | Cone affected | Milder form | Stronger form | Colours that tend to look alike |
|---|---|---|---|---|
| Protan | L (red) | Protanomaly | Protanopia | Reds and greens; reds also look duller |
| Deutan | M (green) | Deuteranomaly | Deuteranopia | Reds and greens |
| Tritan | S (blue) | Tritanomaly | Tritanopia | Blue and green, yellow and red or pink |
The US National Eye Institute (NEI) describes deuteranomaly, the most common type, as making certain greens look more red, and protanomaly as making some reds look more green and less bright; with protanopia or deuteranopia, red and green cannot be told apart at all. In rod monochromacy, also called achromatopsia, no colour is seen, and in blue cone monochromacy only the S cones work.
Red-green deficiency is by far the most common kind, and MedlinePlus notes lower rates than the Northern European figures in almost all other populations studied. Commonly quoted estimates split the roughly 8 percent of Northern European men into about 5 percent of all men with deuteranomaly and about 1 percent each with protanomaly, protanopia and deuteranopia.
Tritan deficiency affects fewer than 1 in 10,000 people worldwide, men and women equally. Blue cone monochromacy affects about 1 in 100,000.
The difference between the sexes is genetic. The genes for the L and M cone pigments, OPN1LW and OPN1MW, are on the X chromosome, and red-green deficiency is inherited in an X-linked recessive pattern. A man has one X chromosome, so one altered copy is enough; a woman has two and usually needs both to be altered. Tritan defects come from a different gene, OPN1SW, and are inherited in an autosomal dominant pattern, which is why they are equally common in both sexes.
Colour vision can also change during life: eye diseases, some medicines and a clouding lens can all affect it, so a new change is a reason to have your eyes examined.
The best-known screening test was published in 1917 by Shinobu Ishihara, a Japanese ophthalmologist. Each Ishihara plate is a circle of coloured dots of varied sizes and shades, some of which form a number. On most plates, the colours of the number and the background are picked so that someone with a red-green deficiency sees them as alike, while the varied dot sizes and shades stop the outline from showing through in any other way.
The full edition has 38 plates, and shorter editions are common. The plates come in several designs:
The Ishihara plates were designed to screen for red-green deficiency and do not test for tritan defects. Eye-care professionals add other tests when needed, such as plate sets with blue-yellow plates, cap-arrangement tests and the anomaloscope, the reference instrument for red-green deficiency, which can tell protan from deutan and anomalous trichromacy from dichromacy.
Printed plates use carefully controlled inks and are meant to be viewed in good daylight or under a lamp that imitates it. On a screen, the test maker controls none of that:
An online result can therefore be wrong in either direction: a mild deficiency can pass, and a person with typical colour vision on a tinted screen can miss plates. A result that suggests a deficiency is a reason to book a colour vision test with an optometrist or ophthalmologist, who can confirm it, name the type and judge how strong it is.
The Color Blind Test is a 12-plate screening for red-green deficiency in the style of Ishihara plates. The plates are generated in your browser, not copied from the originals. Each one has about 1,500 non-overlapping dots of random sizes, and the number's tilt and position shift slightly from plate to plate. Dot positions never depend on the number, and every dot is made randomly lighter or darker by up to about a quarter, so neither size nor brightness gives the number away; only hue does. The two colour sets on a test plate lie along one line, chosen as a compromise between the protan and deutan confusion lines.
The plates always come in the same order:
| Plate | Kind | Typical colour vision | Typical red-green deficiency |
|---|---|---|---|
| 1 | Demonstration (blue on orange) | Reads the number | Reads the number |
| 2 to 8 | Transformation | Reads the number | Sees nothing, or on three plates only one digit |
| 9 to 11 | Fainter number ("Hidden number" in the results) | Reads the number | Sees nothing |
| 12 | Control (violet on khaki) | Reads the number | Reads the number |
There are three sets of numbers, and a retake uses new dots and a different set. You type the digits you see, up to three, or press "I can't see a number", and there is no Back button. The result follows fixed rules:
For example, someone who reads plate 1, sees nothing on plates 2, 4, 6 and 8 to 11, sees only one digit on plates 3, 5 and 7, and reads the control scores 2 of 12: a possible deficiency, labelled strong. The result page also lists your answers beside what most people see and what is often seen with a red-green deficiency, offers a one-line result to copy, and links to the Color Blindness Simulator. The plates are drawn and scored in your browser, and nothing you type is sent anywhere.
The page states its limits too. It does not test for tritan deficiency, it cannot tell protan from deutan, and the mild, moderate and strong labels describe your answers, not a measured severity. It needs a browser that can draw on a canvas and cannot be completed with a screen reader. Before starting, switch off night mode and colour filters and wear your usual glasses.
Inherited colour vision deficiency has no cure, and many people only find out they have it when they take a test. Tinted glasses sold for colour blindness can make some colours stand out more for some wearers, but they do not restore typical colour vision. Reading labels, noting which position is lit on a traffic signal and asking for colour names all help.
For anyone who makes charts, maps, interfaces or slides, the main rule is never to use colour as the only way to carry information. The Web Content Accessibility Guidelines (WCAG) make this success criterion 1.4.1, Use of Color. Add labels, patterns or icons, and check contrast. The Color Blindness Simulator shows an image as it might look with protanopia, deuteranopia, tritanopia or achromatopsia, the Contrast Checker tests text and background colours against WCAG, and the Color Palette Generator builds palettes and checks their contrast.
What is the most common type of colour blindness? Deuteranomaly, the milder form of deutan (green-cone) deficiency, which makes some greens look redder. Commonly quoted estimates put it at about 5 percent of men of Northern European descent, more than all the other types combined.
Why are men more often colour blind than women? The genes for the red and green cone pigments are on the X chromosome. Men have one X, so a single altered copy causes red-green deficiency, while women usually need two. That is why it affects about 1 in 12 men but 1 in 200 women of Northern European ancestry.
Can an online color blind test diagnose colour blindness? No. It can only screen, because screens, settings and lighting vary and nobody supervises the test. A result that suggests a deficiency is a reason to see an optometrist or ophthalmologist, who uses calibrated tests under controlled light.
Does colour blindness get worse over time? Inherited red-green deficiency generally stays the same throughout life. A change in colour vision is more likely to have another cause, such as an eye condition or a medicine, and is worth having checked by an eye doctor.
What does the ArrayKit Color Blind Test check? It screens for red-green deficiency with 12 generated plates in the style of Ishihara plates and gives one of three outcomes: no sign, possible (mild, moderate or strong) or inconclusive. It does not test blue-yellow vision and cannot tell protan from deutan.
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