The ONLY , and I mean ONLY way I've found to stop the greying (and I'm talking magnolia since I haven't used much holly, but the theory should apply all the same) is that you need to rough out the bowl or cut the pen blanks YESTERDAY.
Pen blanks should dry fine. Bowl blanks are way too thick to trust, so I'd rough out the bowl and give it a good alcohol soak overnight to kill off any fungus and to more rapidly dry the bowl. Just soak it overnight and take the usual precautions when drying, but it should dry in about half the normal time.
FYI, I alcohol soak ALL of my green pen blanks. I got some green (really, really green) manzanita root from Nolan-- I sliced them up (1" square by however long) and soaked in methanol (careful, it's toxic stuff, dna works fine). Not a single one of them cracked more than just a fuzz around the corners (next time I'll block plane the corners off).
I can assure you that if you follow what I just said, and use the alcohol soak, the blanks stand a VERY high chance of staying white.
And if they do turn out good, remember me when you divvy them up[
].