The short answerColour blindness, more accurately colour vision deficiency, means the eye cannot tell some colours apart the way a typical eye can. The most common type is difficulty telling red from green. Most cases are inherited through the X chromosome, so it affects men far more often than women. It is easily picked up with Ishihara plates. The inherited type stays the same for life and has no cure, but it does not affect sharpness of vision, so a person can still see 6/6.
What happens in the eye?
The retina has three types of cone cells, each most sensitive to part of the light spectrum: red, green or blue. The brain compares signals from all three to work out a colour. If one type is weaker than normal or missing, certain colours get confused. Most people with colour blindness still see colour but mix some shades up. Seeing only in black and white is very rare.
Types of colour blindness
| Type | What the person notices | How common |
|---|---|---|
| Red-green | Confuses red and green, and sometimes brown, orange and purple | By far the most common, mostly inherited |
| Blue-yellow | Confuses blue with green, and yellow with pink or grey | Rare, can be inherited or caused by eye disease |
| Complete colour blindness | Sees in shades of grey, usually with poor vision and strong light sensitivity | Very rare |
Why is it more common in men?
The genes for red and green colour vision sit on the X chromosome. Men have one X, so a single faulty copy causes colour blindness. Women have two, so they are only affected if both copies are faulty. More often they are carriers who pass the gene to their sons. As a result, red-green colour blindness affects roughly 8% of men of Northern European descent and about 0.5% of women, and the figures vary between populations.
How is colour vision tested?
The best-known test is the Ishihara plates: circles of coloured dots with a number hidden in a different colour. Someone with normal colour vision reads the number, while someone with colour blindness reads a different number or sees nothing. The test is done in good daylight, wearing glasses if you use them, and ideally one eye at a time.
- Ishihara plates: quick and good for picking up red-green weakness, but they do not detect blue-yellow problems well.
- Colour arrangement tests: you put coloured discs in order of shade, which helps show the type and severity.
- Specialist instruments: sometimes used for an exact diagnosis, especially in occupational testing.
- Phone and online tests: useful only as a first look, because screen colours vary between devices. They do not replace an exam.
Acquired colour blindness: when is it a sign of disease?
Inherited colour blindness affects both eyes equally and never changes. Acquired colour loss can start at any age, may affect one eye or differ between the eyes, and changes over time. Causes include:
- Optic nerve disease, such as optic neuritis, where colours look washed out, usually in one eye.
- Glaucoma in its later stages.
- Retina and macula disease, including diabetic retinopathy.
- Cataract: colours look faded or yellowish, and many people notice the difference once it is removed.
- Some medicines taken over a long period, which is why doctors sometimes check colour vision regularly during treatment.
A new change in colour vision, especially in one eye, or with blurred vision or pain when moving the eye, needs an eye exam soon.
Is there a cure? And what about jobs?
There is no cure for inherited colour blindness at present. Tinted glasses and lenses can increase contrast between some colours for some people, but they do not restore normal colour vision. Acquired colour loss may improve if the cause is treated. Most people with colour blindness live normally and learn to rely on brightness, position and labels instead of colour.
Some jobs require normal colour vision or test for it, such as aviation, some rail and maritime roles, electrical wiring, and some military and police posts. Rules differ between countries and employers. Testing a child early helps before choosing a field of study, and telling teachers means classwork does not rely on colour alone.
Frequently asked questions
Does colour blindness get worse with age?
The inherited type stays the same for life. If you notice a new change in your colour vision, it usually means an acquired cause in the eye or optic nerve, and you should have an exam.
Can girls be colour blind?
Yes, but much less often, because both copies of the gene must be faulty. Most women in families with colour blindness are carriers without being affected themselves.
Do colour blind glasses cure colour blindness?
No. They can make some colours stand out more for some people, but they do not restore normal colour vision or treat the cause.
Are online colour blindness tests accurate?
Only as a rough first check. Screen colours and brightness vary, so a diagnosis needs printed plates in proper lighting at an eye clinic.