Colour vision: the same page, seen differently

Most people see colour with three kinds of light-sensing cell, or cone, tuned to long, medium and short wavelengths, roughly red, green and blue.

When one kind is missing or works differently, some colours that look distinct to most people look the same. The common forms affect red and green: protan (the red-sensing cones) and deutan (the green). They come in a milder form, where the cone is shifted, and a full form, where it is missing. Tritan, which affects blue and yellow, is rare, fewer than 1 person in 10,000, and affects men and women equally. Seeing no colour at all, achromatopsia, is rarer still, about 1 in 30,000, and comes with sensitivity to light and poor sharpness.

So the old phrase “colour blind” is misleading. Almost nobody with it sees in black and white. They see a world in which certain pairs of colours, often red and green, or green and brown, collapse into one.

Red-green colour vision deficiency affects about 1 in 12 men and 1 in 200 women¹ of Northern European descent.

It is less common in most other populations: about 4 to 6.5% of Chinese and Japanese men, for example. The genes sit on the X chromosome, which is why it runs through families on the mother’s side and is so much more common in men. A figure of 300 million people worldwide is often quoted, though no single study stands behind it.

In one survey, nearly nine in ten⁵ people with the full form said colour got in the way of everyday tasks, and up to six in ten had trouble with colour-coded charts. People describe anxious art classes as children, and doors that closed later: “I wanted to be a pilot, but when I failed the colour vision test, I felt like a door had closed,” one told researchers in 2025⁶.

In 1794 the chemist John Dalton described his own colour vision to a learned society in Manchester.

A green cloth looked to him like “dull, dark, brownish red”, and a laurel leaf matched red sealing wax. He guessed the fluid in his eyes was tinted blue, and asked that they be examined after he died. They were clear. His eye was kept, and in 1995 scientists read its DNA and found he had been green-blind⁷, not red-blind as everyone had assumed for 150 years.

In 1875 two trains collided at Lagerlunda in Sweden and nine people died. A professor, Frithiof Holmgren, blamed colour blindness and used the disaster, with what two historians call “some unashamed subterfuge”⁸, to win colour testing for every railway worker. The crash itself probably had other causes. In 1917 the Japanese ophthalmologist Shinobu Ishihara published his plates of coloured dots, still the best-known test today.

A simulation shows which colours get confused. It does not show what anyone actually experiences.

Ours uses colour matrices of the kind published by Machado, Oliveira and Fernandes⁹ in 2009, which build on work by Brettel, Viénot and Mollon in 1997. They show the full forms; most people have a milder one, and colour vision also changes with lighting and with age. Use it to find the places where your design leans on colour alone, then check with people who see this way.

This tool changes the pixels you see, so it works on anything in a browser: a live site, a Figma prototype link, a Penpot or Sketch prototype, a Storybook story or a generated app.

Open the prototype, click the bookmark, and click through it. The earlier you catch a problem, the cheaper it is to fix.