Colors are all part of the visible spectrum of light, but how you see those colors depends essentially on 2 things: Whether the color is being absorbed/reflected or whether it is observed directly.
In additive color, colors are formed when light of different wavelenghts are overlapped, with all colors overlapping resulting in "white" light.
In subtractive color, colors are formed when light bounces off a surface and you see the light that is reflected, which is what remains after other parts of the spectrum is absorbed. The mixing of all colors will result in the opposite of additive, that is you end up with black.
More information can be found at:
http://en.wikipedia.org/wiki/Subtractive_color
http://en.wikipedia.org/wiki/Additive_color
