fwiw, i run only linux at home (ok, one mac i use on occasion), and have good fun with gimp. its quite mature and sparkly-featured.

as for the color shifting you see when you move an app from "focus" (active), that is what xwindows does when you have low number of colors (8-bit, 256 colors). it gives *all* the palette to the app (i.e. gimp) so other apps turn funny. its freaky-trippy, but in the end gives you all 256 colors to work with in, say, gimp. i dont know if ppc linux supports more than 256 colors unfortunately(?).