What is Colour Blindness?

Colour blindness, more accurately called colour vision deficiency (CVD), is a condition in which a person’s ability to distinguish between certain colours is reduced or absent. Despite the common name, most people with colour blindness are not entirely blind to colour — they simply perceive certain hues differently from people with typical colour vision.

It is one of the most common genetic conditions worldwide, affecting an estimated 300 to 350 million people globally.

How Common is Colour Blindness?

Colour blindness is far more prevalent than many people realise:

~8% of men (approximately 1 in 12) have some form of colour vision deficiency
~0.5% of women (approximately 1 in 200) are affected
This means around 1 in 12 of your male colleagues or classmates likely has CVD
Rates vary slightly by ethnicity, with higher prevalence in populations of Northern European descent

The marked difference between men and women is due to the genetic basis of the most common forms of CVD.

What Causes Colour Blindness?

The Biology of Colour Vision

Human eyes contain two types of photoreceptors: rods and cones. Rods are responsible for vision in low light, while cones are responsible for colour perception.

The human retina contains three types of cone cells, each sensitive to a different range of wavelengths:

L-cones (Long wavelength) — most sensitive to red light (~560 nm)
M-cones (Medium wavelength) — most sensitive to green light (~530 nm)
S-cones (Short wavelength) — most sensitive to blue light (~420 nm)

Normal colour vision (trichromacy) depends on all three cone types functioning correctly. Colour blindness occurs when one or more of these cone types is absent, reduced, or functions with an altered sensitivity.

Inherited Colour Blindness

The most common forms of colour blindness — red-green colour deficiencies — are inherited in an X-linked recessive pattern. This means:

The genes responsible for L-cones and M-cones are located on the X chromosome
Men (XY) only have one X chromosome, so a single defective gene causes CVD
Women (XX) need defective genes on both X chromosomes to be affected, making them far less likely to have CVD
Women with one defective gene are “carriers” who can pass the condition to their sons

This X-linked pattern explains why CVD is roughly 16 times more common in men than in women.

Acquired Colour Blindness

In some cases, colour vision deficiency is not inherited but develops later in life due to:

Eye diseases: glaucoma, macular degeneration, diabetic retinopathy
Medications: certain drugs including hydroxychloroquine and some antibiotics
Toxic substances: exposure to carbon disulfide, styrene, and some other chemicals
Neurological conditions: optic neuritis, multiple sclerosis, Parkinson’s disease
Ageing: the lens of the eye yellows with age, subtly affecting colour perception

Acquired colour blindness can affect one eye or both, and may progress over time.

Types of Colour Blindness

There are several distinct types of colour vision deficiency, classified by which cone type is affected:

Red-Green Colour Blindness (Most Common)

Red-green CVD accounts for approximately 98% of all colour blindness cases. It includes:

Protanopia — complete absence of L-cones (red-blind); affects ~1% of males
Protanomaly — L-cones present but with reduced sensitivity; affects ~1% of males
Deuteranopia — complete absence of M-cones (green-blind); affects ~1% of males
Deuteranomaly — M-cones present but shifted in sensitivity; affects ~5% of males (most common form)

Blue-Yellow Colour Blindness (Rare)

Tritanopia — complete absence of S-cones (blue-blind); very rare, affects both sexes equally
Tritanomaly — S-cones present but with altered sensitivity

Complete Colour Blindness (Very Rare)

Achromatopsia — complete absence of cone function; the person sees only in shades of grey. Affects approximately 1 in 30,000 people

What Does Colour Blindness Look Like?

Many people wonder whether colour blind individuals see the world in black and white. The answer for the vast majority is no.

Most people with red-green colour blindness can still see many colours. The difference is that some colours that appear distinct to people with normal vision may appear similar or identical to a person with CVD.

For example:

Red and green may look like the same shade of brown or grey
Blue-green and grey may appear similar
Purple and blue may be difficult to distinguish

The best way to understand this experience is to use a colour blindness simulator, which can transform any image to approximate how it would appear with different types of CVD.

How is Colour Blindness Diagnosed?

Several tests are used to screen for and diagnose colour vision deficiency:

Ishihara Test — the most widely used screening test; uses pseudo-isochromatic plates with numbers hidden in dots of different colours
Farnsworth-Munsell 100 Hue Test — a more comprehensive test that measures the ability to arrange colour chips in order of hue
D-15 Dichotomous Test — a fast screening tool using 15 colour discs, often used for occupational screening
Anomaloscope — the gold standard for clinical diagnosis, uses a device that mixes red and green light

You can take free online versions of the first three tests on this site, though online tests are screening tools only and cannot replace a formal clinical evaluation.

Is There a Cure for Colour Blindness?

There is currently no cure for inherited colour blindness. However:

Coloured filter lenses (e.g., EnChroma, Pilestone) can help some people with red-green CVD distinguish more colours. They do not “cure” CVD but can enhance colour contrast in certain situations.
Gene therapy research is ongoing and has shown promising early results in animal studies
Acquired colour blindness may improve if the underlying cause is treated

Living with Colour Blindness

Colour blindness is generally a manageable condition that does not affect overall quality of life for most people. However, it can impact:

Choosing careers that require accurate colour vision (e.g., aviation, certain military roles, electrical work, medicine)
Everyday tasks such as reading colour-coded maps, traffic lights, or data visualisations
Social situations such as matching clothing colours

Many people develop effective strategies for dealing with CVD, including memorising the order of traffic lights, labelling clothing, and using apps that identify colours.

Frequently Asked Questions

Can women be colour blind? Yes, though it is much rarer. Approximately 0.5% of women have red-green colour blindness. Women are more commonly carriers of the gene without being affected themselves.

Is colour blindness a disability? In most countries, severe colour vision deficiency qualifies as a disability under relevant legislation, and accommodations may be required in educational and workplace settings.

Can colour blindness worsen over time? Inherited colour blindness is stable throughout life. Acquired colour blindness can worsen if the underlying condition progresses.

Will my children inherit colour blindness? If you are a colour-blind man, your daughters will be carriers. Your sons will not be affected (unless their mother is also a carrier or colour-blind). If you are a carrier woman, each son has a 50% chance of being colour blind.