Actually, there is something important all these answers are missing. Color is determined by the response of the human eye, not by energy or frequency. In order to get the full range ('gamut') of colors, I need a mix of red, green and blue light (hence the RGB displays) and the primaries can themselves all be different frequencies. That is, one RGB system can have one frequency for the red, while another has a somewhat different frequency for red, the only hard and fast requirement being that both of them choose that frequency from somewhere in the red range. But the choice affects the gamut.
Now I said "human eye", but of course, other animals see colors, too. Bees see colors into the ultraviolet. But of course, we have no idea what the ultraviolet colors look like to them, only that they do see them, and can distinguish shades of them.
Wikipedia has a lot of good further info on this, but it is scattered among several articles. Probably http://en.wikipedia.org/wiki/Color_theory#Color_abstractions is the best starting point. For something much more thorough and technical, see Poynton's excellent Color FAQ at http://www.poynton.com/ColorFAQ.html