We all know that any color can be the result of a combination of three primary colors (Red, Green and Blue). Is there any reason why the number of primary colors is three and not two or four?
Apparently, it is our genetics that determines the number of opsin types (and cone types) in human eye. In other living beens the number of opsins, and therefore the number of primary colors, may be different. Some birds may have 4, most mammals - only 2. (Article Evolution of Colour Vision in Vertebrates