Although many computers/monitors now have the facility to use 24-Bit colour (16.7 million), web designers tend to limit themselves to around 256 standard colours such that all monitors running 8-Bit colour will 'see' the pages as the designer intended.

The links below take you into colour theory. The first link explains colour cube theory, the second and third links are designed to make colour choice easier by displaying the Netscape Javascript colour names.