Here's an easy way to make a skull look like a fossil that came out of a tar pit or peat bog - just take any cleaned degreased skull (you don't even need to peroxide it before hand) and soak it in a solution of clothing dye for a period of time. I had a wolf skull and soaked it in brown (it was a liquid Rit dye called Espresso, but you can use powdered dye and mix colors) dye in hot tap water for about 24 hours. The water starts out hot but of course cools off. You could likely speed the process by keeping the water warm with a heater if you wanted to. But you have to check it every twelve hours or so, because some skulls get real dark and others take a few days. This one went pretty dark in a short period of time - I've had some take 3-5 days, though. This is a good way to hide stains if you have a skull that you just can't get a stain out of. This skull didn't seem to have any problems that peroxide wouldn't handle; I just thought it would look cool. But I have done this in the past to cover stains on skulls. After you dye the skull to the color you want (keep in mind it will lighten slightly when it's dry) just dry it out and then seal it with a glossy clear coat. I used Krylon crystal clear on this one.