Colour blindness, more accurately called colour vision deficiency, is grouped into three main types according to which cone cells in the retina are affected: protan (long-wavelength, "red" cones), deutan (medium-wavelength, "green" cones) and tritan (short-wavelength, "blue" cones). Protan and deutan types together make up red-green colour blindness, by far the most common form, while tritan (blue-yellow) deficiency and complete colour blindness are rare. Most people with a colour vision deficiency still see many colours; they confuse particular pairs that look clearly different to others.
- Normal colour vision uses three cone types; colour blindness happens when one type is missing, shifted or, very rarely, when cones barely work at all.
- Words ending in -anomaly (milder, one cone type shifted) differ from those ending in -opia (one cone type missing).
- Deutan types are the most common; protan types also make reds look darker; tritan types confuse blues with greens and yellows with pinks or violets.
- Inherited red-green deficiency is far more common in men because the genes sit on the X chromosome; it is present from birth and does not get worse.
- A change in colour vision later in life can be a sign of an eye or health condition and should be checked by an optometrist or doctor.
How does colour vision work?
The retina has rods, which work in dim light and do not distinguish colour, and cones, which work in daylight and come in three types:
- L cones (long wavelength), most sensitive to yellowish-green light and the main contributors to seeing red, often called "red" cones;
- M cones (medium wavelength), most sensitive to green light, often called "green" cones;
- S cones (short wavelength), most sensitive to violet-blue light, often called "blue" cones.
The brain compares the signals from all three, and the ratio between them is what you experience as a hue; this is called trichromacy. If one cone type is missing or responds to slightly the wrong wavelengths, some colour pairs produce the same ratio and look alike. Colours that look identical to someone missing a given cone type lie on a confusion line.
Anomalous trichromacy, dichromacy and monochromacy
The names for colour vision deficiency combine the cone type affected with how severely it is affected.
| Category | What is happening | Names | Effect |
|---|---|---|---|
| Anomalous trichromacy | All three cone types are present, but one has its sensitivity shifted towards another | Protanomaly, deuteranomaly, tritanomaly | Milder and highly variable; some people barely notice it, others are almost as affected as a dichromat |
| Dichromacy | One cone type is missing or not working | Protanopia, deuteranopia, tritanopia | Colour vision relies on two cone types, so a whole range of colour pairs becomes indistinguishable |
| Monochromacy | Cones barely work, or only one type works | Achromatopsia (rod monochromacy), blue cone monochromacy | Little or no colour vision, usually with reduced sharpness, sensitivity to bright light and involuntary eye movements |
"Colour blind" is the everyday term for all of these, but most people with deuteranomaly, the most common form, see colour much closer to typical vision than to black and white.
Protan, deutan and tritan explained
Protan (red cones affected)
In protanomaly the L cones are shifted; in protanopia they are missing. Reds, greens, browns and oranges are confused, and because L cones provide much of the sensitivity to deep red light, reds also look darker, so a red warning on a dark background can look dim. Purple can look like blue, and some pinks look grey.
Deutan (green cones affected)
In deuteranomaly the M cones are shifted; in deuteranopia they are missing. Greens, yellows, oranges, reds and browns are easily confused, especially when small, similar in lightness or seen in poor light. Reds do not usually look darker. Deuteranomaly is the most common deficiency of all.
Tritan (blue cones affected)
In tritanomaly the S cones are weak; in tritanopia they are missing. Blue is confused with green, and yellow with pink, violet or light grey. Inherited tritan deficiency is rare and, because its gene is not on the X chromosome, affects men and women about equally. Blue-yellow changes are more often acquired than inherited.
| Type | Cone affected | Colours commonly confused | Inheritance |
|---|---|---|---|
| Protanomaly / protanopia | L ("red") | Red with green, brown and orange; purple with blue; reds look darker | X-linked, much more common in men |
| Deuteranomaly / deuteranopia | M ("green") | Green, yellow, orange, red and brown; beige with light green | X-linked, much more common in men |
| Tritanomaly / tritanopia | S ("blue") | Blue with green; yellow with pink, violet or light grey | Not X-linked; rare; men and women about equally |
| Achromatopsia | Cones barely function | Most colours; sees mainly lightness differences | Rare; inherited from both parents |
Protan and deutan confusions are similar and hard to separate without specialist equipment, so many screenings, including ours, report a "protan/deutan" (red-green) result.
How common is colour blindness?
Red-green colour vision deficiency is much more common in men than in women. A commonly cited estimate is about 1 in 12 men (around 8%) and 1 in 200 women (around 0.5%) of Northern European ancestry, with lower figures reported in many other populations. Deuteranomaly is usually reported as the most frequent type.
Inherited tritan deficiency is rare, with widely varying published estimates, and achromatopsia is commonly cited at roughly 1 in 30,000 people. Treat any single figure as an approximation.
Inherited vs acquired colour vision deficiency
The genes for the L and M cone pigments sit next to each other on the X chromosome. Men have one X chromosome, so one altered copy is enough to cause a red-green deficiency. Women have two, so they usually need the altered gene on both to be affected; a woman with one copy is a carrier and typically has normal or near-normal colour vision. A father cannot pass red-green deficiency to his son, because sons inherit his Y chromosome, but all of his daughters will be carriers. Each son of a carrier mother has a one-in-two chance of being affected.
Inherited deficiency is present from birth, affects both eyes equally and does not change; many people only discover it through a school test or job medical.
Acquired deficiency develops later, can affect one eye more than the other and can change. Causes include cataract (a yellowing lens dulls blues), glaucoma, diabetic eye disease, macular degeneration, optic nerve conditions and some medicines, such as ethambutol and hydroxychloroquine, which doctors monitor for. As a rough rule many acquired defects are blue-yellow, while optic nerve conditions often affect red-green, with many exceptions.
How is colour blindness tested?
Professionals combine several kinds of test under standardised lighting.
- Pseudo-isochromatic plates. Circles of coloured dots in which some dots form a number. Figure and background differ in hue along a confusion line, while brightness varies randomly so lightness gives no clue. The Ishihara plates, first published in 1917, are the best-known red-green set; others, such as the Hardy-Rand-Rittler (HRR) test, also screen for tritan defects and grade severity.
- Arrangement tests. You order coloured caps by hue from a reference cap. The Farnsworth D-15 (15 caps) separates mild from more marked deficiencies; the Farnsworth-Munsell 100 Hue test measures fine discrimination. The error pattern points to a protan, deutan or tritan axis.
- Anomaloscope. You mix red and green light to match a yellow. The mixtures you accept classify red-green deficiency and separate anomalous trichromacy from dichromacy. It is the reference standard, mostly found in specialist clinics.
- Occupational tests. Some regulators and employers require lantern tests or computer-based colour tests.
Our colour blind test follows the same principles with dot plates generated by our own algorithm (not copied from any printed test) and a 15-cap arrangement task. Screens and lighting vary, so it is a screening, not a diagnosis; an optometrist can confirm the type and severity.
Living with colour vision deficiency
There is currently no treatment that restores normal colour vision in inherited deficiency, but most people adapt well, and small changes make daily life easier.
- Use other cues. Traffic lights have a standard order, and many cables and controls carry labels or shapes as well as colours.
- Use your devices. Built-in colour filters and colour-identifier camera apps help with clothes, paint and ripe fruit.
- Speak up at school and work. Ask for labelled charts and maps; undiagnosed children can be wrongly thought to struggle with learning.
- Check career requirements early. Roles such as commercial pilot, some electrical trades, train driver and some armed forces and police roles have colour vision standards.
- Be realistic about special glasses. Tinted "colour-blind glasses" can increase contrast between some colours for some people, but they do not restore normal colour vision.
- Driving. In many countries colour vision deficiency alone does not prevent an ordinary licence, but rules differ, so check yours.
How to make charts and designs colour-blind friendly
Colour choices matter for anyone making slides, dashboards, maps or websites. The Web Content Accessibility Guidelines (WCAG) require that colour is not the only visual means of conveying information.
- Never rely on colour alone. Add text labels, icons, patterns, line styles or shapes so meaning survives without colour.
- Avoid red-versus-green pairs. For "good/bad" or two-series charts, blue and orange are a much safer combination.
- Vary lightness, not just hue. Strong lightness differences survive for almost everyone, even in greyscale.
- Label directly. Put series names next to the lines or bars instead of in a separate colour-keyed legend.
- Use a tested palette. Palettes designed for colour vision deficiency, such as the widely used Okabe-Ito palette or the viridis colour scale, avoid the common confusion pairs.
- Check your work. Colour vision simulators are approximations but catch obvious problems.
To check other parts of your vision, try the online eye test or read our guide on how to test your eyesight at home. More screenings are on the health tests page.
Frequently asked questions
What is the most common type of colour blindness?
Deuteranomaly, a mild-to-moderate red-green deficiency in which the green (M) cones are shifted, is usually reported as the most common type. Red-green deficiency as a whole is far more common than blue-yellow deficiency or complete colour blindness.
What is the difference between protanopia and deuteranopia?
Both are red-green dichromacies. In protanopia the red (L) cones are missing, so reds also look darker; in deuteranopia the green (M) cones are missing and reds keep their usual brightness. The colours confused overlap a lot, so the two can be hard to tell apart without an anomaloscope.
Can women be colour blind?
Yes, but inherited red-green deficiency is much rarer in women because they usually need the altered gene on both X chromosomes. A commonly cited estimate is about 1 in 200 women of Northern European ancestry, compared with about 1 in 12 men. Tritan deficiency affects men and women about equally.
Can colour blindness get worse over time?
Inherited colour vision deficiency stays the same throughout life. If your colour vision changes, it is likely to be acquired, for example through cataract, glaucoma, other eye conditions or medicines, and should be checked by an optometrist or doctor.
Can colour blindness be cured?
Not at present for inherited forms. Tinted glasses can change contrast between some colours for some people but do not restore normal colour vision. Acquired changes sometimes improve when the cause is treated, for example after cataract surgery.
Is an online colour blind test accurate?
It is a useful first screening, but screens, colour filters and lighting change how colours appear, so it cannot diagnose a deficiency. Professional tests use calibrated plates or instruments under standard lighting and can confirm the type and severity.