All colours in HTML, CSS, and XHTML are defined using what is called a 6 digit Hexidecimal Tuplet - don´t let the term scare you off, it´s actually quite simple. The colour is defined using a group of three (the tuplet) (three becasue we build colours using reg, green, and blue) values expressed in hexidicimal (base 16)

Thus: if I want to express white which is made be adding all the red, green, and blue together, I would write ffffff

f in base 16 = 15, a=10, b=11, c-12, d=13, e=14, and f=15 - thus the allowable digits are 0-f.

The above example ffffff can be split into three groups of two characters ff ff ff corresponding to Red, Green, and Blue components respectively.

To explain this by example here are a few other colours:

000000 = black (no red, green, or blue)
00ff00 = green (only green)
ff0000 = red
0000ff = blue

Other colours are created using combinitions of the various components. For more information see this page which displays all allowable colours and their definitions:
http://www.pawprint.net/designresources/web-colours.php