Face replacement

Probably a stupid question, but I'm new to the GIMP and haven't yet found the answer anywhere.

I'm doing some simple photo montages, and am pasting friends' faces onto different bodies. I have no problems with the basics, i.e., coping and pasting, rescaling, etc.

But what happens when the face I'm pasting on someone else's body has a different skin tone/colour? How can I get it to blend in with the skin tone of the body?

I've tried selecting and then changing colour, but this doesn't work. I can get the exact colour I want using the eyedropper tool, but I can't then apply it without painting over the eyes, nose, mouth etc. I can change the skin green or blue without problems, but can't get it to blend in with the skin of body.

All help gratefully appreciated!!

Florio