I've found, that much of the purpleheart lumber I buy, isn't as purple as I hoped it would be. I was looking for a wood that was a much more striking vision of purple.

Here's what works for me:

If you get your purpleheart lumber, or pen blanks, or even veneer, and it's on the brown side, set it outside for a few hours, and let the sun hit it.  Or, first try simply leaning it against the wall in your shop, uncovered.  Give it plenty of air circulation.  Typically, in one day I'll notice the purpleheart has changed a deeper color...from brown to purple.

Then, to keep it there, after you create your pen, or build your object with the wood, use a good ultraviolet blocker when you finish the wood, and it should stay that color for a very long time....if not forever.