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Actually it's to do with your post count, just like the "rank" above them.
As you can see, you're a Vibrating Furby whilst I'm a Whalloping Walrus and Oddi's an Avid Aardvark.
As your post count increases, these change too. The color is different for moderators and admins too, matching their current position. Vibrating Furby is the "new" rank and changes to Avid Aardvark after 10 posts I think it is then it changes every 100th post in alphabetical order
