You could possibly try vinyl dye (in a spray can), but the colours are fairly basic. I'd also consider the iMac plastics to not be a good candidate, as they're quite hard and brittle I couldn't imagine the dye to "take" very well. Still, perhaps if you can practive the dying in a non-visible area, I'd consider it.

Another option would be to apply an adhesive vinyl wrap - there are lots of places around that do this using a vinyl printer (fairly common nowadays), for car windows and the like. You can also buy the stuff yourself from hobby stores, it's sold for wrapping the wings of RC planes.

JB