Understand how each type of color vision deficiency works, what colors are affected, and how common it is - then take a free online screening test for a first indication.
Color blindness - more accurately called color vision deficiency (CVD) - is a reduced ability to tell certain colors apart. It is rarely 'blindness' to color in the literal sense; most people with CVD see colors but confuse specific pairs such as red and green. CVD affects roughly 1 in 12 men (about 8%) and 1 in 200 women (about 0.5%) worldwide, so most people know someone who has it, often without realizing it.
There are three broad families of color vision deficiency: red-green (by far the most common), blue-yellow (rare), and complete color blindness (very rare). Each is caused by a different problem with the light-sensitive cone cells in the retina, and each has a 'dichromacy' form (a cone type is missing) and a milder 'anomalous trichromacy' form (a cone type works but is shifted).
This guide is educational. The Ishihara test on this site is a screening tool, not a medical diagnosis - it gives a first indication of a possible red-green deficiency. Only a qualified optometrist or ophthalmologist can confirm the type and degree with clinical tests.
Normal color vision relies on three types of cone cells in the retina, each tuned to a different part of the light spectrum: L-cones (long wavelengths, ~560 nm, 'red'), M-cones (medium, ~530 nm, 'green'), and S-cones (short, ~420 nm, 'blue'). The brain compares the signals from these three cones to build the full range of colors we perceive - this is called trichromatic vision.
Color vision deficiency happens when one cone type is missing (dichromacy) or shifted so its sensitivity overlaps too much with a neighbor (anomalous trichromacy). Then certain colors send nearly identical signals to the brain and become hard to tell apart. Which colors are confused depends on which cone is affected - and that is what defines the types below.
Red-green deficiencies are by far the most common and are caused by problems with the L-cones or M-cones, whose genes sit on the X chromosome. Because men have only one X chromosome, they are affected far more often than women.
About 1% of men
Protanopia is a red-green deficiency in which the L-cones ('red' cones) are entirely missing. People with protanopia struggle to distinguish red from green, and reds appear noticeably darker than normal - a bright red can look almost black or dark brown. Because red light appears dim, they may confuse red with black, dark green or dark brown, and have trouble seeing red warning lights or brake lights at a distance. Purple can look like blue because the red component is not perceived.
About 1% of men
Protanomaly is the milder form, where the L-cones are present but shifted toward green sensitivity. Reds, oranges and yellows look duller and greener than they should, and reddish colors are harder to tell apart in low light. Many people with protanomaly manage everyday life without realizing they see color differently.
About 1% of men
Deuteranopia is a red-green deficiency in which the M-cones ('green' cones) are missing. Green is perceived as beige or tan, and red and green are easily confused. Unlike protanopia, brightness is closer to normal, so the main difficulty is hue rather than darkness. Greens, browns, oranges and reds can all blur together, which makes tasks like reading colored charts or judging the ripeness of fruit harder.
About 5% of men - the single most common type
Deuteranomaly is the mild form, where the M-cones work but are shifted toward red sensitivity. Reds and greens look more similar than they should, especially in muted or pastel shades. Most people with deuteranomaly have only mild difficulty and often pass everyday tasks without noticing - which is exactly why a screening test can be revealing.
Blue-yellow deficiencies affect the S-cones. Their gene sits on chromosome 7 (not the X chromosome), so this type affects men and women about equally and is much rarer than red-green.
Very rare, well under 0.1% of people
Tritanopia is a blue-yellow deficiency in which the S-cones are missing. Blue and green become hard to separate, and yellow can appear as light gray or violet. It is often acquired later in life - from aging, eye disease or injury - rather than inherited.
Extremely rare
Tritanomaly is the milder blue-yellow form, where S-cones function but are shifted. Blues and greens, and yellows and pinks, are harder to distinguish, though the effect is subtler than in tritanopia.
The rarest and most severe forms, in which color is barely perceived or absent entirely.
About 1 in 30,000 people
In complete achromatopsia the retina has no working cone function, so the world is seen only in shades of gray. It is usually present from birth and often comes with other symptoms such as light sensitivity (photophobia), reduced sharpness of vision, and involuntary eye movements (nystagmus). Milder 'cone monochromacy' leaves a single working cone type. Because it involves more than color, anyone who suspects achromatopsia should see an eye care professional.
A quick comparison of the main types, the cone affected, and how common each is.
| Type | Cone affected | Colors confused | Prevalence | Severity |
|---|---|---|---|---|
| Protanopia | L-cone missing | red/green; red looks dark | ~1% of men | Strong (dichromacy) |
| Protanomaly | L-cone shifted | red/green (mild) | ~1% of men | Mild |
| Deuteranopia | M-cone missing | red/green; green looks tan | ~1% of men | Strong (dichromacy) |
| Deuteranomaly | M-cone shifted | red/green (mild) | ~5% of men | Mild - most common |
| Tritanopia | S-cone missing | blue/green, yellow/violet | <0.1% | Strong (rare) |
| Tritanomaly | S-cone shifted | blue/green (mild) | very rare | Mild |
| Achromatopsia | no cone function | all colors (sees gray) | ~1 in 30,000 | Total (very rare) |
Most color blindness is inherited. The genes for the L- and M-cones (red-green vision) are located on the X chromosome and are passed down in an X-linked recessive pattern. Men have one X and one Y chromosome, so a single altered gene on their X causes the deficiency - which is why red-green CVD is roughly 16 times more common in men. Women have two X chromosomes, so they are usually only affected if both carry the change; more often they are unaffected 'carriers' who can pass it to sons. Blue-yellow deficiency is inherited differently (chromosome 7) and affects both sexes equally.
Color blindness can also be acquired rather than inherited. Aging, eye conditions such as glaucoma, cataracts and macular degeneration, diabetes, multiple sclerosis, and certain medications (for example some drugs for tuberculosis or rheumatoid arthritis) can all reduce color perception - typically blue-yellow. Unlike inherited CVD, acquired deficiency can change over time and may affect one eye more than the other, which is a reason to have any sudden change in color vision checked by a professional.
The best-known screening method is the Ishihara test, created by Dr. Shinobu Ishihara in 1917. It uses plates of colored dots with a number hidden inside; people with normal color vision read one number, while those with a red-green deficiency read a different number or none. It is quick, widely used, and very effective for screening red-green deficiencies.
The free test on this site is a digital Ishihara screening. It can give you a fast, private indication of whether you may have a red-green color vision deficiency and roughly how strong it is. It is not a clinical diagnosis. On-screen results depend on your display's color calibration, brightness and ambient light, and a standard Ishihara test mainly detects red-green (not blue-yellow) deficiencies.
If a screening suggests a deficiency - or if you need a result for a job, license or medical reason - see an optometrist or ophthalmologist. A professional can confirm the exact type and degree using calibrated Ishihara plates plus additional clinical tests such as an anomaloscope and color-arrangement tests (D-15, Farnsworth-Munsell 100 Hue). Think of an online screening as the first step, and a professional exam as the diagnosis.
Deuteranomaly - a mild 'green-shifted' red-green deficiency - is the most common, affecting about 5% of men. Together, red-green deficiencies make up the large majority of all cases.
No. '-opia' means a cone type is missing (dichromacy, a stronger deficiency); '-anomaly' means the cone is present but shifted (anomalous trichromacy, usually milder). Deuteranopia = M-cones missing; deuteranomaly = M-cones shifted.
Inherited color blindness cannot currently be cured, but it usually does not worsen and most people adapt well. Special filter glasses and smartphone apps can improve color separation for some people. Acquired color vision problems may improve if the underlying cause is treated.
Complete achromatopsia (seeing only in gray) is the rarest and most severe, at about 1 in 30,000. Blue-yellow (tritan) deficiencies are also rare compared with red-green.
Most cases are inherited through an X-linked gene, which is why men are affected far more often than women. It can also be acquired from aging, eye disease, diabetes, or certain medications.
Yes - the large majority of people with red-green deficiency drive safely. Traffic lights follow a fixed position (red on top), which helps. A few occupations with strict color standards, such as commercial pilots or some rail roles, do have restrictions.
Yes, but it is much rarer - about 0.5% of women, versus 8% of men - because women have two X chromosomes. Many women are unaffected carriers who can pass red-green CVD to their sons.
An online Ishihara screening is a reliable first indication for red-green deficiencies, but accuracy depends on your screen's color calibration, brightness and lighting. It is a screening, not a diagnosis - confirm any result with an eye care professional.
Inherited color blindness is stable and does not progress. Acquired color vision loss can change over time, so a noticeable shift in how you see colors is worth having checked.
An optometrist or ophthalmologist confirms it with calibrated Ishihara plates plus tests such as an anomaloscope and arrangement tests (D-15, Farnsworth-Munsell 100 Hue), which identify the exact type and severity.
This page is for education and awareness. The online Ishihara test is a screening tool that indicates a possible color vision deficiency - it is not a medical diagnosis and does not replace an eye examination. For a definitive diagnosis, or for any occupational or medical requirement, consult a qualified optometrist or ophthalmologist.
July 2026