In the eye are many small light sensing cells called photoreceptors. There are 3 different types of photoreceptors involved in sensing colour, each one reacts more strongly to one wavelength of light than other wavelengths. However, as you know, there are more than three different colours around, so how does that work?
Your brain combines the information from the activities of the three different photoreceptor types, and from this information is able to figure out what colour it is. People who are colour blind have a missing or damaged photoreceptor type. With only two working types of photoreceptors, they confuse some colours which most people would be able to tell apart. Colour vision defects affect about 8% of males, and 0.5% of females, and are inherited through the female line. Jobs where colour vision is important include pilots, police officers, artists and designers.