Now that you mention color, the inside of the mouth would just be open and you'd see through that (it's a skeleton). So the grey would be that light brown color. If this was on a white shirt, there would be no reason not to have white teeth too.
I attached the pic of a couple of changes that I think make it more appealing to me overall. Hope you don't mind. It's just quick color changes to help express my view.
Great feedback, and I get your point on the see through of the mouth. I noticed you took off the yellow border as well on the inside plaque, that seems to help with the transition from light to dark brown.
Thanks, I'm not a designer by any stretch. I just thought that it seems logical to take the yellow out given the 4 color constraint and to help convey that it's on a plaque..
edit:
Another idea would be to use a more traditional shape for the plaque.