What turtle is named for the color of fat under its shell?

That would be the 'Dermochelys pokemonus'. More commonly referred to as the Blastoise Turtle.

I cannot answer the question above but I can unanswer it by saying the above answer is a joke. There is no such species as pokemonus in the genus Dermochelys, except maybe in the game of Pokemon. The only living species in the genus Dermochelys is a large sea turtle often called a leatherback, Dermochelys coriacea, and it does not have a shell, but instead has a thick leathery skin resembling a shell.