I think I am semi qualified to answer this question. When purple heart is cut or sanded or planed it turns a brownish color thats really far from purple. To get the purple color back, let it sit for a week or two it turns dark purple through oxidation and sun light. So, as time goes along it becomes a darker purple which is the ultimate.
For the neck blank, think of purple heart as maple on steroids. Its stronger, harder, more dense, and brighter. You should have no problem using the piece either quartersawn or flatsawn without carbon rods. I generally do multi lam necks just because the wood I buy is generally flatsawn and I make my laminations and turn em quartersawn so that I have a three piece neck. And with an ebony board these necks will never move its great.
If you are using the purple heart as a top piece, be careful not to remove too much with the router at one time because, a) you could destroy your carbide router bits, first hand experience, the router could kick back at you, and c) tear out the wood.
Hope that helps
MzI