Here are six test tubes filled with coloured dyes. How many different colours do you see? Most people say six, but some people would say only two or three. There are even some (very rare) people who see no colour at all.
How can it be that one person says two things have the same colour, yet somebody else says they are completely different?
Colour isn’t really there
Scientists know that Isaac Newton did more than just sit around watching apples fall into his garden. He linked gravity on Earth to the movements of planets, and his experiments with glass prisms showed that white light is a mixture of different wavelengths (he called them refrangible rays).
One of his many brilliant insights was that unlike size or weight, colour is not a property of the objects that fill our world. Colour depends precisely on which wavelengths of light are bounced from objects before they reach the eye.
Colour is a sensation, a property of the mind. No matter how bright and vivid and real they seem, colours are inside your head, not outside.
Why do people see colours?
Light is picked up in the eye by three types of cells called cone cells. They are called cones because in the microscope they look like tiny ice-cream cones.
They are nicknamed red, green, and blue because they pick up different wavelengths and there are millions of each type cone cells. Just as a painter can mix from three tubs of paint to produce a wide and vivid palette, your brain uses these three cone types to create thousands of colour sensations.
What causes colour blindness?
The cone cells are just like other cells in the body - they are controlled by genes. The genes controlling cones are prone to faulty replication during early development, and affected cones either fail to develop, or start to pick up abnormal wavelengths. The result is like taking away or diluting one of our painter’s tubs: the colour sensations are reduced or changed.
We can filter an image to show how a red-green colour blind person might see it; colour does not disappear but the range of colours is reduced.
How is colour blindness inherited?
Every cell in every woman’s body contains two gene packages called X chromosomes, but men have only one. The genes controlling red and green cones are located on X chromosomes.
If a woman has a faulty or missing gene on one X chromosome, the gene on the second X chromosome works as a backup and the cones develop normally.
But if the faulty X chromosome is transmitted from mother to son, there’s no backup, and the son will have reduced or altered colour sensations, called red-green colour blindness. Other forms of colour blindness are much more rare and are usually more severe.
Can colour blindness be cured?
Most scientific studies suggest that the wiring of the eye and brain is identical in people with normal and abnormal colour vision. The only difference is at the first stage of vision, where the cones can be faulty.
The obvious solution is to fix the faulty cones, and this is what a team led by scientists Maureen and Jay Neitz at the University of Washington in Seattle have attempted.
Monkey see, monkey do.
Maureen and Jay Neitz study a species of monkey in which all the males are red-green colour blind. In gene therapy trials, their team injected colour-blind male monkeys with the gene controlling the missing cone type.
We don’t know what monkeys see, but we can see what they do. Two monkeys tested so far learnt how to tell red from green patterns after the injections.
These are promising results but the eye is a very delicate organ and the injections are still dangerous to sight. Colour blindness may be curable but there is still some way to go.
See more Explainer articles on The Conversation.