Colour Blindness: Types, Causes, Tests and Daily Life Tips

Colour blindness, more accurately called colour vision deficiency (CVD), means seeing certain colours differently from most people or finding some shades hard to tell apart. Very few people with it see the world in black and white. Most can see plenty of colours but confuse particular pairs, typically reds, greens, browns and oranges, or less often blues and yellows. Many people only discover it as adults, when a colour-coded chart, a cable or a ripe-or-unripe banana suddenly makes the difference obvious.

This guide explains how colour vision works, the main types of deficiency, why it is much more common in men, how it is diagnosed, how it affects school, work and driving, what tinted glasses such as EnChroma can and cannot do, and when a change in colour vision is a sign of eye disease that needs attention.

How colour vision works

The retina contains two kinds of light-sensitive cells. Rods work in dim light and do not distinguish colour. Cones work in daylight and come in three types, each most sensitive to a different part of the spectrum: long-wavelength (L) cones, roughly red; medium-wavelength (M) cones, roughly green; and short-wavelength (S) cones, roughly blue. The brain compares the signals from these three cone types to create every colour we perceive. Our guide to how vision works explains this process in more detail.

When one cone type is missing or its light-sensitive pigment is shifted in sensitivity, the comparison changes and some colours become harder to separate. Whether a colour looks different depends on which cone type is affected and how severely.

Types of colour vision deficiency

Doctors describe colour vision deficiency by the cone type involved and by severity. The suffix -anomaly means the cone is present but altered (milder); -anopia means the cone type is effectively absent (more severe).

Red-green deficiency (protan and deutan)

Together, protan and deutan defects make up almost all inherited colour vision deficiency. People with mild deuteranomaly may only notice problems with subtle shades or in poor light, whereas those with protanopia or deuteranopia confuse many everyday colours. In protan defects, red light appears dimmer, so red text on a dark background or a distant red light may be harder to see.

Blue-yellow deficiency (tritan)

Inherited tritan defects are rare and are carried on a non-sex chromosome, so they affect men and women equally. Blue-yellow problems are more often acquired, for example from eye disease, than inherited.

Achromatopsia

Complete achromatopsia is a rare inherited condition in which none of the cone types work. As well as seeing no colour, people have reduced sharpness of vision, strong sensitivity to bright light (photophobia) and often nystagmus, a rhythmic shaking of the eyes, from infancy. It is a different and more disabling condition than ordinary colour blindness and is managed by low-vision and genetic specialists; see our guides to nystagmus and low vision.

Why men are affected more: X-linked inheritance

The genes for the red and green cone pigments sit on the X chromosome. Males have one X and one Y chromosome, so a single faulty copy of the gene on their only X chromosome causes colour vision deficiency. Females have two X chromosomes, so they usually need faulty copies on both to be affected; with one copy, they are carriers with normal or near-normal colour vision.

This is why colour blindness often appears to skip a generation, passing from a grandfather through his daughter to a grandson. Figures vary by ancestry: rates are lower in many African, East Asian and Indigenous populations than in people of Northern European origin. Our article on family history and eye disease risk explores inheritance more broadly.

How colour vision is tested

Ishihara plates

The Ishihara test is the best-known screening tool. It shows a series of plates made of coloured dots, with a number or a winding line hidden in dots of a different colour. People with normal colour vision see one number, while people with red-green deficiency see a different number or none at all. It is fast and widely used in schools, clinics and occupational medicals, but it is designed to detect red-green defects, does not reliably detect tritan defects and does not precisely grade severity.

Other tests

• Anomaloscope. The person adjusts a mixture of red and green light to match a yellow reference. The settings reveal the type and severity of red-green deficiency precisely. It is considered the reference standard but is mainly found in specialist centres.
• Arrangement tests, such as the Farnsworth D-15 and the Farnsworth-Munsell 100 Hue, ask the person to place coloured caps in order. They help grade severity and detect blue-yellow problems.
• Hardy-Rand-Rittler (HRR) plates screen for both red-green and blue-yellow defects and give an idea of severity.
• Lantern tests are used by some transport and maritime authorities to simulate signal lights.

You can try our online colour vision test as a first check. Screen colours vary between devices, so online tests cannot give a diagnosis; if you suspect a problem, ask an optometrist for formal testing. To see how the world might look to someone with a particular type of CVD, try our colour blindness simulator.

Daily life with colour vision deficiency

Most people with CVD live without major problems, but small adjustments make life easier:

• Labels and patterns. Ask for charts, graphs and maps that use labels, shapes or patterns rather than colour alone. Designers can choose colour-blind-friendly palettes.
• Apps and phone settings. Colour identifier apps name the colour under the camera, and phones and computers offer colour filters and accessibility modes.
• Clothing and food. Organise clothes with labels; use a thermometer for cooking meat rather than relying on colour.
• Traffic lights. Remember the position of the lights. In most countries red is at the top or left. People with protan defects should be aware that red lights may appear dimmer from a distance.
• Health checks. Some may find it harder to notice skin rashes, sunburn, blood in urine or changes in test strips; asking someone else to check can help.

Careers and driving

Most careers are fully open to people with colour vision deficiency. A few jobs have colour vision standards because colour signals are safety-critical. Requirements differ by country and employer and change over time, so always check the current rules before committing to training. Areas where standards commonly apply include:

• Commercial and military aviation (pilots, some air traffic roles).
• Railway drivers and some maritime roles such as deck officers.
• Some electrical work involving colour-coded wiring.
• Some police, fire and armed forces positions.

In many fields, such as medicine, design or photography, people with CVD succeed using workarounds and technology. Ordinary driving licences in most countries do not require normal colour vision, though some countries have specific rules for professional drivers. See our eye care by profession guide for more on job-related vision demands.

EnChroma and other colour-correcting glasses

Glasses marketed for colour blindness, such as EnChroma, use special filters that block narrow bands of light where red and green cone sensitivities overlap. This can make some colours appear more saturated and increase the contrast between certain reds and greens, and some users describe the experience as striking, particularly outdoors.

However, independent studies have found that these glasses do not restore normal colour vision and do not significantly improve scores on standard diagnostic tests such as the Ishihara or anomaloscope. They may help some people with mild to moderate deficiency enjoy colours more, but they will not help someone pass an occupational colour vision test, and they tend to work less well indoors and for people with complete dichromacy. Tinted contact lenses, such as red-tinted lenses worn in one eye, have similar limitations and can reduce depth perception and vision in dim light.

Acquired colour vision problems

Colour vision can also change later in life because of disease, medication or ageing. Unlike inherited CVD, acquired defects often affect one eye more than the other, may change over time and frequently involve blue-yellow colours. Causes include:

• Cataract, which yellows and dims colours gradually; many people are surprised how bright blues look after cataract surgery.
• Glaucoma, diabetic retinopathy and macular degeneration, which can affect blue-yellow discrimination.
• Optic nerve disease, particularly optic neuritis, which characteristically makes colours, especially red, look washed out in one eye.
• Medications, including hydroxychloroquine, ethambutol and some others; see medications that affect the eyes.
• Toxins and nutritional deficiency, such as certain solvents or severe vitamin B12 deficiency.
• Brain injury or stroke, rarely causing loss of colour perception (cerebral achromatopsia).

When to see a doctor

If you think you or your child may have colour vision deficiency, an optometrist can test it in a routine appointment. This is especially worthwhile before choosing a career path with colour vision standards.