I've been trying to figure out what specs get which colours, and if it's at all possible to use another spec's mount. But all I'm seeing is conflicting information everywhere...
On the PTR I've seen Demonology riding purple mounts, Destruction riding both purple AND green, and nobody at all with the orange one. The mount journal states the orange one can be bought from Calydus, presumably if you've already completed the quest (all other class mounts also list vendors to buy certain colour variations from).
So who actually gets what colour? Can you use another spec's mount? Can you only get one mount, or all of them?