Modern computers typically use 3 bytes to produce colour, which can be represented as Red, Green and Blue values each ranging from 0 to 255. This is the system HTML uses (FF is 255 in Hexadecimal notation) and is a fairly standard one.

Using this system we have 256 to the power 3 (or 2 to the power 24) colours to play with. This yeilds a range of 16,777,216 posible colours.

Other colour models in use are Cyan, Magenta, Yellow (CMY)often with black added (CMYK) which is most commonly used in printing, Brightness Hue and Satuaration and Luminance and CIE (International Commission on Illumination) models.

Jon