My initial thought for Xebec's hair color was black and white, which I think looks the best. Black and blue would fit with the Buggy theories, while black and red would match his Jolly Roger design. So, blue, red, or white are the most likely options.
But I'm going with the more interesting dark horse: green. Brook's soul abilities are green-colored, and Zoro's King of Hell has a green aura. If Blackbeard's unusual body is due to an extra soul, this could be a symbolic hint. And the fact that Xebec's debut took place in the Underworld feels telling.