Colour blindness: types, tests and why it mostly affects men

The condition owes its traditional name, Daltonism, to John Dalton, the English chemist who at the end of the eighteenth century described his own difficulty in distinguishing certain colours. About 150 years after his death, genetic analysis made it possible to establish what his defect really was¹. Colour blindness, in the meantime, has turned out to be one of the most common vision disorders³.

The large population surveys indicate that, among people of European origin, it affects about 8 men in 100 and 4 women in 1,000². It is an enormous difference between the sexes, and the explanation lies in a chromosome.

In this article

1. What colour blindness is
2. Why colour blindness mostly affects men
3. Acquired colour blindness
4. In everyday life
5. Glasses for the colour-blind: what the studies say
6. Frequently asked questions
7. Sources

What colour blindness is

We see colours thanks to the cones, cells of the retina that contain light-sensitive pigments. There are three types, sensitive to long, medium and short wavelengths: simplifying, to reds, greens and blues^1,4. The brain compares the signals of the three types and works out the colour from them.

In congenital colour blindness one of these pigments works differently from usual or is missing⁴. In the first case colours are perceived, but some shades are confused; in the second the range of distinguishable colours is reduced further. The main types are:

Type	Cones involved	How common it is
Protan (red-green)	sensitive to long wavelengths	together with deutan, the great majority of cases4
Deutan (red-green)	sensitive to medium wavelengths	together with protan, the great majority of cases4
Tritan (blue-yellow)	sensitive to short wavelengths	congenital, one person in several tens of thousands4
Achromatopsia	no functioning cones	rare1

The red-green forms are milder than achromatopsia, the rare complete loss of colour vision, which does not depend on the pigment genes but on other cone proteins^1,4. In other words, the image of the colour-blind person who sees ‘in black and white’ corresponds only to a very small minority.

Why colour blindness mostly affects men

The genes for the red- and green-sensitive pigments are on the X chromosome, in a row: a red gene followed by one or more green genes. The blue one is instead on chromosome 7¹. The red and green genes are almost identical, 96%, and this similarity favours recombination errors: a gene can be lost or fuse with the other into a ‘hybrid’ gene. This is the mechanism behind most red-green forms^1,5. The retina uses only the first two genes in the row, and how marked the defect is depends, roughly, on how different the pigments they produce are^1,5.

Men have only one X chromosome: if it carries an altered gene, the defect shows. Women have two, and almost always the second compensates³. This is why many more women carry the alteration without showing it, the so-called carriers: according to an estimate based on the frequency in men, between 9 and 15%⁶. A son of a carrier mother can inherit the X chromosome with the altered gene.

The frequency also varies between populations. A review of population surveys found values between 4 and 6.5% in men of Chinese and Japanese origin, lower than in Europe; the review attributes the differences mainly to chance factors in the history of the populations rather than to natural selection².

Acquired colour blindness

Not all colour alterations are present from birth. Colour vision that changes during life can depend on diseases that affect the transparent structures of the eye, the retina, the optic nerve, up to the visual cortex of the brain, or on general diseases⁷. In acquired forms an alteration on the blue-yellow axis is more frequent⁷, so much so that a blue defect suggests first of all an acquired cause, given that the congenital form is so rare⁴.

Telling a congenital form from an acquired one may require several tests: plate tests, hue-arrangement tests, the anomaloscope and sometimes genetic analysis⁴. Plates are a simple screening method, but on their own do not give a diagnosis⁴. How they work and how reliable they are is explained in our article on colour blindness tests.

In everyday life

A 2019 review identified 20 studies on the impact of colour blindness in daily life and grouped them into seven themes: everyday activities, work, choosing products, emotions, school and vocational training, self-care and health, and even some advantages. The most recurrent difficulty concerned work, but the authors stress that the studies are few and concern very specific groups, such as drivers and medical students⁸.

For congenital forms, management today consists mainly of information and guidance, including in the choice of profession³. Some jobs, in fact, have specific requirements for colour vision.

Glasses for the colour-blind: what the studies say

Glasses and contact lenses with coloured filters are sold, presented as aids for people who confuse colours. The available evidence calls for caution.

A 2022 systematic review pooled 16 studies of different types, including comparisons between groups, prototypes, single cases and narrative reviews, on electronic and optical devices for colour blindness, including commercially available glasses and contact lenses. The conclusions: there is no evidence of a clinically relevant improvement in colour perception, although some shades may be perceived differently⁹. A review from 2010 already observed that these aids can help in specific tasks, but do not allow normal colour discrimination to be reached³. For the future, the same review indicated gene therapy as a possibility, on the basis of results in animals³; where research stands for inherited retinal diseases is described in our article on gene therapy.

Frequently asked questions

Can colour blindness be cured?

For congenital forms there is currently no cure: management is based on information and guidance³. The optical aids studied so far do not restore normal colour vision^3,9, while gene therapy, in the 2010 review, was a possibility studied in animals³.

Can women be colour-blind?

Yes, but much more rarely: about 0.4% of European women against 8% of men². Many more are carriers, that is, have the alteration on one of the two X chromosomes without showing it: between 9 and 15% according to one estimate⁶.

Do colour-blind people see in black and white?

Almost never. In the most common forms some colours are confused, while complete loss of colour vision, achromatopsia, is rare^1,4.

Can you become colour-blind as an adult?

Congenital colour blindness is present from birth, but an alteration of colours can appear during life because of diseases of the eye, the nervous system or general diseases⁷. A recent change should be assessed by the ophthalmologist.

Colour blindness is frequent and, in its most common forms, mild^2,4. Knowing about it serves mainly two purposes: understanding why certain colours cause confusion, so as to organise oneself at school and at work, and recognising when an alteration in colours is not the usual one but a new change to have assessed^3,7.