Color vision

Most people see color with three kinds of light-sensing cell in the eye, called cones, tuned to long, medium and short wavelengths of light, roughly red, green and blue.

When one kind is missing or works differently, some colors 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 color at all, achromatopsia, is rarer still, about 1 in 30,000, and comes with sensitivity to light and poor sharpness.

So the old phrase “color blind” is misleading. Almost nobody with it sees in black and white. They see a world in which certain pairs of colors, often red and green, or green and brown, collapse into one. A sign that speaks only in those colors is a sign with no words on it.

Red-green color 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 color got in the way of everyday tasks, and up to six in ten had trouble with color-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 color vision test, I felt like a door had closed,” one told researchers in 2025⁶.

In 1794 the chemist John Dalton described his own color 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 color blindness and used the disaster, with what two historians call “some unashamed subterfuge”⁸, to win color testing for every railway worker. The crash itself probably had other causes. In 1917 the Japanese ophthalmologist Shinobu Ishihara published his plates of colored dots, still the best-known test today.

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

Ours uses color 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 color vision also changes with lighting and with age. Use it to find the signs on your page that have no words, only color, then check with people who see this way.