To specify colors in Web pages, you can refer to the predefined set of colors by name. For more control, though, you have to use a six-digit hexadecimal code, specifying the mix of red, green, and blue (RGB) in each color. (Here's a handy hex-to-decimal conversion chart, if ever you need one.)
Think of each color code as three sets of two hexadecimal digits: the first two for red, the second two for green, the last two for blue. Any of these three can be assigned a value from 00 to FF (hex for 255), meaning you have a theoretical limit of 256 x 256 x 256 colors, or 16.7 million -- more than the human eye can distinguish. For practical technical reasons (i.e., not all monitors can show them), it's best to stick to numbers like 00, 11, 22, . . . EE, FF. That still gives you 16 x 16 x 16 colors, or 4096. (It's better still if you stick to 00, 33, 66, 99, CC, and FF.) Some samples:
Your colors don't all have to be 00 or FF: you can use different numbers between them to get different shades. The lower the numbers, the closer to black.
Someone has collected a palette of the 216 most reliable colors to use on Web pages.
You can put the color tags in several places: