What is colour blindness?
The eye determines the colour of what we are looking at by how much response comes from each of three types of light receptors (red, green, or blue). The most common type of colour vision deficiency is where one type of light receptor, usually red or green, doesn't work as well.
This reduces the range and appearance of colours, with some colours that are clearly different to normal eyes appearing the same to a colour deficient people. What do they see? Have a look at this example of colour blindness. Here is another example of colour blindness.
True colour blindness - black and white only - is very rare. Colour vision deficiencies are usually inherited, and are more common in boys than girls.