What they are are ranks. Purple is for Hylian Knights and green is for Hylain Nobles. The rank is acquired by post count, post quality, how long you've been a member here at ZD, reputation, and lastly, 75% approval from all Hylian Knights, Nobles, and admins that vote for you.
Other colors:
Orange= Moderator
Blue= Super Moderator
Red= Admin
Cool people have my color. Seriously, you're good to go man. Everyone else just sucks so bad, that they get noticed and given responsibility. Whats up with that?