The most reliable way to specify a colour in HTML or style sheets remains the hexadecimal code. This is because any Web browser, even an old one, is extremely likely to recognize the hexadecimal notation as a colour code. Some Web browsers may not recognize the newer notations used to define colour or the most recent set of named colours. Outside of the standard colour names, there are a wide variety of names used for colours.

For more information visit:
Defining Colour
Hexadecimal Color Codes

The last link also has a very useful colour guide where you can grab the colours for yourself.