Understanding colour blindness

What colour vision deficiency is, how common it is, and why contrast alone isn't always enough.

What is colour blindness?

Colour blindness — more accurately called colour vision deficiency (CVD) — is a condition where a person cannot perceive certain colours or colour differences the way most people do. It is usually inherited and affects the cone cells in the retina.

Most people with CVD are not truly “blind” to colour. Instead, they see a narrower range of colours and may confuse colours that look distinct to others.

Types of colour vision deficiency

Protanopia (red-blind)

Affects approximately 1% of men

No functioning red cone cells. Reds appear dark and muddy. Red and green can be indistinguishable. Oranges, yellows, and greens may all shift toward yellow.

Deuteranopia (green-blind)

Affects approximately 1% of men — the most common severe form

No functioning green cone cells. Similar to protanopia in practice — reds and greens are confused. However, reds don't appear as dark as they do with protanopia.

Tritanopia (blue-blind)

Very rare — affects less than 0.01% of the population

No functioning blue cone cells. Blues and greens are confused. Yellows can appear as light pink or grey. This type affects men and women equally.

Achromatopsia (total colour blindness)

Extremely rare — approximately 1 in 30,000 people

No functioning cone cells at all. The world is seen entirely in shades of grey. People with achromatopsia often also have reduced visual acuity and sensitivity to bright light.

How common is it?

Colour vision deficiency is more common than many people realise:

Approximately 8% of men (about 1 in 12) have some form of CVD.
Approximately 0.5% of women (about 1 in 200) are affected.
In a team of 25 people, statistically at least one person is likely to have colour vision deficiency.
In an audience of 1,000, around 40 men and 2-3 women will have some form of CVD.

Why contrast isn't always enough

A good contrast ratio ensures that text is readable in terms of brightness difference. But two colours can have adequate contrast and still be indistinguishable for someone with CVD.

For example, a red button on a green background might have a passing contrast ratio, but someone with red-green colour blindness may not be able to tell the button apart from its background at all.

This is why accessible design follows two principles:

Sufficient contrast — use this tool to verify your ratios meet WCAG thresholds.
Never rely on colour alone — always pair colour with text labels, icons, patterns, or other visual cues to convey information.

Practical tips for designing with CVD in mind

Use icons and labels alongside colour indicators (e.g. a tick icon with “Pass”, not just a green dot).
Avoid red/green pairings as the sole way to distinguish states (e.g. success vs error).
Use patterns or textures in charts and graphs to differentiate data series, not just colour.
Test your designs with a colour blindness simulator — our colour blind mode is coming soon.
Choose a palette with sufficient lightness variation between colours, not just hue variation.