"White face" is often used to refer to an ashen look, indicating fear or dismay. Since they're daringly trying to rescue Buckbeak in this scene, Rowling could be referring to the blood draining from her face from fear. But even I would call that a bit of a stretch. She wrote seven books, the last four of which were lengthy. I'm sure that among all of the words she wrote, it's easy to forget one adjective.
I'm pro-black Hermione. Who cares, anyway? A series this successful will probably be interpreted a thousand different ways for years to come. Let it be.