Colour does not exist! There is no intrinsic quality of any spectral interval of EM radiation that has a colour.
Our human colour vision is the resulting stimulus of different spectral luminances on the RGB sensitive cones in the eye. The brain interprets these differences in spectral stimulus as 'colour'. Dietmars explanation of course is far fuller.
Human females can see far more hues than human males as the genes for colour vision are on the x chromasome.
To show how important the relative spectral intensities are and the environment they are in, google Land Effect. Edwin Land was the inventor of instant photography (Polaroid) and some others.
Here is a very good explanation of the Land Effect.
http://www.aw3rd.us/scief/colorviz.htm
This effect also works with two filters of the same colour but different spectral content. The effect is weaker but still there.
Most people cannot believe their eyes and suspect trickery when this is done with two slide projectors.
It is also the basis for some colour optical illusions.
Bert