I did the same with the window screen.
My bear skull turned the acetone brown... Not sure thats good.
Normal, don't worry about it.
I suspect that your acetone is saturated (or nearly saturated) with fat and is not degreasing now (or at least not very well) Get some fresh acetone is all you need to do.
To get around the problem of acetone becoming saturated with fat, museums and universities have automatic degreasers that spray acetone on the bones in a closed system, and then collect the used (greasy) acetone, then distill it. It is then sprayed again over the bones being degreased. These machines work amazingly well, but they are expensive because they have to be designed to safely contain boiling acetone and have a LOT of failsafe mechanisms to cope with the huge fire hazard! :-)
John
