Imo it's got something to do with the new Vampire face skin textures, and adding that to already exisiting mods from before Dawnguard tends to confuse things. It's been trial, and error for me, but from what I've played with thus far I found that if you go into CK, and try to match up the skin complexion with the base head part, (something like match Skinheadnordmale with Baseskinheadnordmale) and change it from when it's usually being listed as DEFAULT, then export the face, it can snap back into matching the body. I use cbbe, and better males, and I've managed to fix most of the vampires I have come across with discoloration. Also sometimes tinkering with the actual vampire race itself, and setting a certain skin as default, and then going to the npc, and exporting the textures, can solve the problem.