I don't notice dramatic tonal differences on fretted Alembic fingerboards, and I've never heard Purpleheart on a fretless, so I'd have to say I don't know, V.
What I do notice is a different tactile response. Purpleheart is very dense (not as dense as Ebony), and has a slightly open grain compared to Ebony. I can describe it as feeling a little drier than Ebony.
MD is right, the Purpleheart darkens over time, mostly from exposure to UV rays. Under the polyester finish it should always look purple, even as it darkens up. As a fingerboard or under an oil finish, it can eventually look nearly brown. Of course, as soon as you refret it, the shock of purple color will return.