I've never seen the sapwood on purpleheart, but the dark lines in the purple make me think either kingwood, or some really fresh East Indian Rosewood. I guess it could also be Bois de Rose, with mostly sapwood. I would say with almost certainty, that it is sapwood, with a little heart. There is a wood called palisandra (a faux rosewood, I believe), or something like that, that comes to mind, but I don't know it well enough to make a guess.
No matter what it is, it's a nice pen. I've never come accross deep yellow sapwood. It's quite nice.
Rob