CRT iMacs? Swap the plastics. IIRC they all used the same molds, or at least something compatible. Other than those colors tho, it would have to be painted. I think the plastics were molded with the dye in them, so short of recasting parts (not unheard of but extremely expensive) you're out of luck with changing the plastics. I don't know if thats the case on the pattern iMacs but why anyone would own one of those is beyond me.

On the snow, and possibly the graphite you could hit it with a transparent enamel on the outer surface and get the same general effect. Any hobby shop that carries Testor's model paints should have some of the transparent stuff. The occasional walmart carries some of the transparent line as well, but I've only seen the transparent red at walmarts.

If you're willing to give up transparency then you could hit it with regular paint and some wax on the outside. If you've got a graphite you could get a decent effect using a flat paint on the inner surface, but you would get a grey tint.

For silver there are a number of metalic paints and glazes out there. Experiment with those on an old case and you could find something that works for you.