If you ever want to know what colors signify which faction, I have your answer! This will help with decoding member signatures (which are where a member lists the active characters on their account) and writing your own!
Alliance - Blue
Neutral with focus on Alliance - Dark Blue
True Neutral - Gray
Neutral with focus on Horde - Dark Red
Horde - Red
Lich - Cyan
These are just the basics, but if you would like to add a faction, comment and we will add it!