I'm considering switching my tauren druid to a troll druid when it's possible to do so, but I wasn't sure if I could have a troll I liked with forms I liked (they're a bit... bright...).
So I borrowed the beta from a friend of mine enough to take a poke through the starter troll quests, and got a troll druid to lvl15 (when you get bearform)... and promptly ran to the barber shop and tested which hair colors gave which druid form colors.
No, I've no idea why purple hair returns a white-and-dark-blue bear and cat and dark blue hair returns a greenish-and-cyan bear and cat. It's trolls, who knows why they do what they do. :P Might've made more sense to have the dark blue hair return the white-and-dark-blue forms, at least, but not my decision.
This may still change, but that's how it is in beta right now.
To be honest I'm not sure if this information can already be found elsewhere -- if it can, I'll feel silly because I looked and could find nothing. ¬_¬
- Troll druid form color chart