At the back of the eye in the retina, you have receptors that enable you to see called cones and rods. Rods allow you to see black and white and cones allow you to see colour. There are three types of cones, green detecting, red detecting and blue detecting. For more information please see the how the eye works section.
There are Three main versions of colour blindness. A red-green colour deficiency, blue-yellow deficiency and total colour blindness. Red-green deficiencies are the most common form of colour blindness. About 8% of men and less than 1% of women have a red-green deficiency. A red-green colour deficiency occurs when a person has an absence of red cones (formally called deuteranopia) or green cones (formally called protanopia). Their remaining 2 cones are still able to see all the other colours, they may just get confused with green and red. There are different degrees of severity, some people may not be able to tell green and red apart and others will be able to.
People with a blue-yellow deficency have an absence of blue cones. This is quite a rare form of colour blindness.
Total colour blindness occurs when a person has no cones, only rods and can see only in black and white. This form of colour blindness is extremely rare.