I wouldn't know first-hand. I've only ever used GIMP, and not PS, but I doubt it would be very dissimilar. If you have a basic grasp of layers, you should be good to go.
If you want any in-depth advice on the program, just PQ me and I'll see if I can't find my artist to lend a hand. As one example of what she has colored digitally, check this out
here on DA . All coloring, shading, and the snow were done entirely in the GIMP, using basic techniques, nothing very drastic or tough.
But like I said before, if you want to really get into the GIMP, you're going to have to be willing to do a little experimentation on your own as well as following advice and tips. :)