I boiled the head of a deer I shot this year. Used Dawn dish soap. Got it clean pretty easy. Ordered some 40% peroxide off amazon that took a couple weeks to show up. While I was waiting the skull started turning a bright yellow in spots. Applied the peroxide and it whitened it up nice. Now a few weeks later the yellow is coming back. Any advise to get it to stay white?

My thought is when boiling it can cause the fat to leach deeper into the bone and is now coming back out. The beetle cleaned skulls I sell I usually soak in dawn and water for several days before whitening them. Or several weeks if it is a hog skull. Using a bucket heater or aquarium heater in the dawn mixture can speed the degreasing, but since I do this to help cover the cost of my leases, I don't want to spend more money on electricity than the profit I make selling the skulls so I usually just give it more days pouring the grease off the top and extra dawn/water changes. Some people will also soak them in acetone to degrease them for much faster and I am sure better results and is probably what I should be doing with my hog skulls.

Use borax while simmering, you don't really want a rolling boil. add as soon as it starts to simmer, and keep adding every 30-45 min. It'll make the meat turn to a jelly like texture and just kinda slip right off.

Adding ammonia to the degreasing mix will really help your results. Aquarium heaters work ok but you really have to get the temp close to 120 to fully degrease, on some models you can manipulate the thermostat to get them that high.

What your seeing is left over fat and grease in the skull from not being fully degreased. My advice is to simmer it (no boil) for as long as you can. I have an industrial style tank and heater set up with a thermostat to keep it bt 120 and 125 and my minimum degrease time is a week on deer.

I use a few drops of Dawn and 1-2 scoops of Oxy powder. They turn out nice and white without using anything else.

More Dawn will work if that is what you have, simmer it for several hours and change the water if it gets scummy looking. It needs to be degreased more thoroughly as stated.