First of all you have to distinguish between the human perception of 'red' or 'green' light and the scientific measurable properties of what we call 'red' or 'green'.
Visible light of all colors has an associated wavelength and a frequency, related by
lambda = c / f where lambda is the wavelength, c is the speed of light, and f is the frequency. These properties are measurable in the laboratory.
The human ability to distinguish one 'color' or wavelength has to do with the properties of the eye. There are several types of light sensing cone photoreceptors in the retina that are sensitive to different wavelengths. The brain receives information from these three types and then figures out what the color is.
What we perceive as 'red' tends to have longer wavelengths than that of 'green'. So it tends to excite one type of photoreceptor more than the others. The brain decides "It's Red"
Look at the Wikipedia site http://en.wikipedia.org/wiki/Color_vision
for more information.
p.s.- and come back with a follow-up if more discussion is needed. Mike W.
(published on 10/22/2007)