How to paint open mouths on larger figures?

Well, if this were a human, I'd suggest a redder caucasian flesh color. You could darken it towards the back and lighten it towards the front and edges. You might want to try to speckle the gums or paint them brown, if you want a challenge and if they gums are visible.

Don't paint the teeth white. Go with an ivory color for the highest value and fade towards a brownish tan to get a more animalistic effect, possibly with some striations.

Same with the sclera (whites of the eyes). Use a pale flesh, Since the creature is humanoid, and presumably dark skined, try shooting for a pinkish effect. I'm in the process of painting a buffalo soldier bust and did this with the eyes. It reduced the contrast, added a more lifelike look and avoided a cartoonish (and potentially racist) popeyed look. Let the pupils be partly, about half, covered by the eyelids. Don't paint them completely round. Try to add a pin point of white/off white to the pupil for a catchlight. That adds a lot of life to the face.

   Sounds like good advice.   I did a Google search for some pics of gorilla mouths, and then thought of what my dog's mouth looks like, so I think I'm headed in the right direction.    Thanks!

     Everybody's gonna laugh their heads off when they see what I do with this kit.  Gonna be awesome  hehehehehehe.

I did the same kind of thing with the Godzilla kit.  I used blends of pink and a little flesh, and black (for gum lines, like a dog's) to paint the mouth.  And Al's right--don't use white for teeth.  I used Cream from Testor's enamel line (the little square glass bottles) for the teeth.