This is a great example why you should not boil a skull. There are many downsides to boiling skulls, cracking and flaking are big ones. The bone acually changes after it's been boiled and becomes brittle. Often the process does not remove enough grease and what is left behind comes to the surface over time and yellows or grays the bone. The delicate nasil tissue is lost and often meat is left inside the skull to rot. Another major downside to boiling your own skull is the hassel, you put hours into cleaning it yourself and it wont last.
Here is a cougar skull that was boiled many years ago, it is gray and there is yellow tissue still left on the skull. It was acually greasy to the touch. After I properly degreased the skull I was able to remove all the old tissue that had yellowed. I then whitened and sealed the skull. I think the results are great and now this skull can proudly be displayed again!