Anyone have an idea of why it would happen or how I can stop it/reverse it?

it is embedded grease that is just now leaching out of the skull. Was the skull boiled/simmered? It most likely can be fixed by putting it in a degreasing bath for some time and then rebleach it.

X2

You can tell it was boiled as the nasal bones are all gone. Looks to be boiled very badly.

I honestly think the greasy yellow tint will be in that skull no matter what. A few degreasing baths could help but that skull will never be white.

Dang it.

Thanks for the help. Maybe a bleaching would work also?

spray paint

Mud Shark... I'm not the most knowledgable person on skulls, but we have soaked some really greasy skulls in white gas for a few days and on some it got all the grease out and some it didn't.

This may be a dumb question, but what is white gas?

Coleman fuel

Coleman fuel works well. We used to use it on pig skulls and it did the job. Took more then just a few days though. I'd give it a shot. You can't damage the skull anymore by trying it. Just don't smoke around it.;)

Get your self several large bottles of peroxide from Wal-Mart, wrap the skull in medical gauze and soak in a large pot of the peroxide for a day, you can wire a sick to the altlers to keep them out of the peroxide. remove the gauze and rinse well. Should be fine.

That may be part of the problem. We just moved and it was stored in my in-laws garage. My F-I-L smokes like a train, in the garage when it's cold...

I could be wrong but I think he was referring to not blowing yourself up.

He is saying don't smoke around the coleman fuel. Smoking around the skull will not turn it yellow.

x2 that's what I was referring to.

it looks like you have boiled it a lot but if you boil it with dawn for about 15 minutes it usually helps.

You may want to look for something called trophy bleach paste. This is a product that can be applied as a paste with a brush, and when allowed to dry can be brushed off to reveal a bone white appearance.

Sometimes it does not remove the patina (could be yellowing due to smoke or particulates in the air that have attached to the skull and oxidized) but it is inexpensive and does not require you to remove the trophy from the plaque. Just a thought.

Definately grease.
Coleman fuel will work...but you have to be patient...our big Brown Bears soak for MONTHS...not days....

I heard you can cook the grease out in an oven. Any truth to that?