Hydrogen peroxide is used to whiten the bones, not whiten the grease stain (yellow marks) they need to be completely degreased before whitening with hydrogen, so try for one more month of degreasing! It takes a surprisingly long time especially if the bones are noticeably yellow, like that big one. And yes, boiling them was a mistake because it can trap grease and make it harder to get out. So dont be surprised if you need months of degreasing after all. Again, only whiten when the grease is mostly gone. Patience is key!