I'm a late comer to this one too. But try this: I turn my purpleheart blanks last thing in the evening before I shut down the shop. Sand ready to finish, and leave it in the lathe. I pull a two lamp fluorescent task light down right above, within inches of the blank, form a makeshift reflector underneath the blank from aluminum foil by wrapping aroung the ends of the mandrel. Then I leave the lamp on overnight. If you haven't tried this, you won't believe the beautiful shade of purple you get. Next evening I apply the finish.
According to the books, It's ultra violet radiation that causes freshly cut prupleheart to turn from brown to purple. The fluorescent lamps put out a lot of UV. Of course, when you read further, It's also UV radiation that ultimately turns the purple back to brown. So, I always tell my customers to expect purpleheart and also osage orange to turn brown over time.