Colour blindness Dr Rob Hicks Colour blindness is the reduced ability to distinguish between certain colours. It's usually inherited and is more common in men, affecting about one in 20. Far fewer women - around one in 200 - are affected.

In this article

What is it? The Ishihara test

Why does it happen? Can I stop it?

Should I see a doctor? Treatments

There are several forms of colour blindness. The most common form is red/green colour blindness, which involves the confusion of red and green. It's passed on through a faulty colour vision gene on an X chromosome as a recessive disorder. This type affects about one in ten men and has two forms:

• different shades of red appear dull and indistinct
• greens, oranges, pale reds and browns all appear as the same hue, distinguished only by their intensity

In one rare form of colour blindness, blues and yellows can't be distinguished. In another, all colours are seen in black and white.

Vision itself isn't affected, only the ability to distinguish between certain colours.

The image on the right shows a background of green dots with two wavy lines made up of red and orange dots. If you're colour blind, you won't be able to see these colours and so you won't be able to pick out the pattern from the dots.

This is the most common test used to diagnose colour blindness. You may also come across a version that uses one colour of dots to spell out numbers that can be read by someone with normal vision, while colour blind people only see a random pattern.

The retina of the eye has colour-detecting vision cells, called cones, which are necessary to see colour properly. There are three types of cone cell, sensitive to red, blue, or green light. If one or more of these types of cells is faulty, then colour blindness results.

Sometimes colour blindness occurs because of diseases such as macular degeneration or from side effects of medicines.

Stop signs

Colour blindness needn't stop someone driving, because traffic lights can be distinguished by the position of the light. However, it can be an obstacle to particular careers where good colour vision is important, including pilots, electricians, train drivers and some jobs in the printing, fashion and design industries.

Not if it's inherited. If it's because of underlying eye disorders or medication then it can sometimes be treated and stopped.

This isn't necessary unless other eye problems, for example blurred vision, are occurring.

As inherited forms of colour blindness are harmless, no treatment is needed.

This article was last medically reviewed by Dr Rob Hicks in April 2007.
First published in October 1997.