The tans and burns are done with additional textures, one for tan and one for burn for every natural skin tone. There's a new version of the TONE file that links up three textures (normal, tan, burn) for every skin. The fantasy skins simply have zeros for the tan and burn links so they're not used. Oh, and there's what I call a burn mask that gives a sort of splotchy appearance with pale areas around the eyes, etc.
I've made a converter that'll update the TONE and automatically generate the new textures, so anyone wanting fantasy skins to tan/burn can clone them and run the converter on them. For natural colors it's pretty simple, just darken for tan and make redder for burn. For unnatural colors I went with finding whether blue and/or green is more dominant than red and emphasizing those colors in the burn. So if the skin is blue you get a blue burn, etc. Made sense to me. :)
