Colour blindness is an inaccurate term for a lack of perceptual sensitivity to certain colors. Absolute colour blindness is almost unknown, but in very rare cases, total colour blindness occurs. Colour blindness can happen in one of two ways:
Typical: Complete inability to discriminate between any colour variations, which is usually associated with other severe vision impairments.
Atypical: The ability to only see very clear colours.
Affecting around 7% of men and around 0.04% women, the main form of colour blindness is the inability to distinguish red from green. This means that people affected do not see the colours red and green the same way as others. The cause of this is due to the red or green photoreceptors working incorrectly.
Rods and Cones
Our eyes contain what are called rods and cones. It is these rods and cones that give us the ability to see colour.
The rods deal with brightness and the cones with colour. There are three different types of cones: red cones - green cones and blue cones. People that are effected by colour blindness, have less numbers of particular cones than normal, so they get colours confused. They may be able to see a bright green coloured object out side, but when viewed in artificial light, the same object may appear brown in colour. Or if the object was a dull green, it could even appear red.
Take a look at the two images below:
People with normal colour vision and those with all colour vision deficiencies should read the number 12.
People with normal colour vision should see the number 8. Those with red-green colour vision deficiencies should see the number 3. Total colour blindness should not be able to read any numeral.