All the grease has to be removed from skull before hydrodipping if not the grease will blead through the dip. The yellowing is grease and most things don't stick well to grease. Just a heads up I do lots of hydrodipping
Use dawn dish detergent and water for degreasing. I bought an aquarium heater that I use while degreasing. Works pretty good it you give it time. when water gets cloudy take skull out and add new water and soap.