Guilds can apply for a unique colour. To do so, your guild must have at least 5 active members on 5 different accounts. Active members are players who put a lot of time and effort into the game. All members of your guild must show above average roleplay at all time.
If you feel your guild can live up to these requirements, you can apply for a colours by contacting a member of the staff. The best way to apply for guild colours is by sending a PM (on the Forum) to a GM.