You can use inks as well, I just water them down to help flow into the cracks. Use a test piece to get the right mixture that you like, I use a 50/50 blend myself. If you are going for the white look try using a dark to medium gray, or for a more natural look a brown wash. For my skulls and bones I do a base coat of Bleached Bone then a wash of brown, let it dry fully and then go over it with a dry bush of Bleached Bone again to bring out the highlights.
Forgot to say that I was taking about the horns and skulls, pretty much any part that is just the pure white.