It is important to understand that colour is as much about Biology as  Physics.  The human eye contains three types of colour sensors, which are sensitive to light energy in three specific bands of frequency.  One of these bands we call green, one blue, one red.  All other colours are the brain's interpretation of the mixture of these three colours reaching the eye.  

If the eye had sensors responding to different frequencies of light, we would perceive colours completely differently - who knows what other species may see that we miss?  Only today, with ultra-violet and infra-red cameras are we beginning to see how for example plants attract insects with displays and patterns that we cannot even see.