It's more complex than just creation date. Somewhere in there it involves votes cast on your posts, which is why you might see someone's name switch colors from

It's more complex than just creation date. Somewhere in there it involves votes cast on your posts, which is why you might see someone's name switch colors from one post to another in the same thread (the system doesn't go back and switch name colors on previously-created posts). IIRC the exact mechanism isn't public.