It seems like you may be getting it too hot, especially if you're buffing it.
Purpleheart gets darker and darker as it ages, until, in some cases (I have a purpleheart tanto-bokken that is about twelve years old) you get to an almost grape-like purple.
I don't know that there's any artificial way to speed that process. If there is, though, I feel certain that somebody HERE will know it! Good luck!