R+G+B = RGB

For most purposes, every colour can be created by combining the three primary colours. For pigment these are red, yellow and blue. A painter will work with these colours to create more colours for her palette. With light the situation is slightly different - green replaces yellow - and these are the primary colours you will use in computer graphics and video.

The combination of red, green and blue in different proportions can describe almost any colour. In practice some colours can not be created, partly because your colours can only be as 'pure' as the red green and blue from which you create them. For this reason you are dependent on the phosphors or LCSs on the face of your monitor ... amongst other things.

Besides RGB, a common way to denote colour is using hue, saturation and brightness (HSB). Hue is colour. The saturation of a colour says how pure it is and basically follows a line between the pure colour and grey where both have the same brightness. So pink is de-saturated red and grey could be totally desaturated anything. The brightness, which is the third parameter, says how far towards full power the three colours are and basically follows a line between white, the pure colour and black with the colours in the same proportion.

I should also mention CIE L*a*b which denotes colour by the luminance (as with HSB) and then two colour components, which, roughly speaking, represent how far the hue is around the colour wheel.

Moving back to red, green and blue ...

In this image you can see that the brightness of the chairs is different in the three smaller black and white images underneath. This shows how much red, green and blue (from left to right) there is in the colour image. You would get the same effect if you took three black and white photographs with deep red, green and blue filters across the lens in turn.