Thanks for the encouragement, QueenOfShinyThings!
(great screenname, btw!)
That picture shows as far as the colors change, from the most brown in the center to the most green on the outer edge, so I think I'm gonna keep on with it and see how it turns out.
ETA: My biggest problem now is that now I've got the hang of it, this is ALL I want to do! And I have an essay due tomorrow!