Thanks for the advice. I actually had forgotten to make it transparent, so I've just done that. You're right, it looks a lot better!

I'll probably get back to it soon enough and make some edits (like the ones you suggested) when I'm finished the current piece I'm working on.

I think it's good that you didn't pixel the inside of the mouth. This reads very well as it is, and detail inside the mouth would've been distracting.

The colours feel a little disjointed. They're all very "pure" (perfectly neutral greys, mostly pure reds) and don't seem like they affect each other or belong together. Try having the greys of the teeth have some pink in them since they're surrounded by red and would reflect some of it, or try adding a hue shift towards the same hue in both the red and grey ramps.

I think the background could be made transparent, since it doesn't seem to have any particular purpose other than to not blend with the mouth itself.