I'd tint the greys so they aren't completely desaturated, gives 'em visual interest. I'm really confused by the volumes right now. not sure why the area bordering the temple is highlighted, and the mouth area is pretty confusing. there's also some banding on the tentacle things and the area where the jaw meets the skull. jaw blade looks to be from side-view, while head is at 3/4ths. if you have a preliminary sketch, it'd be useful in helping us delineate what's what.