Just a suggestion (sorry, no experience with painted computers, but I wonder how to restore the original look, as well): find a motorcycle shop that offers custom painted helmets. ABS is a common material for helmets (as some other plastics and fiber reinforced resins are, as well). So identify the stuff your Mac's case is made of (some kind of ABS, I assume) and ask for the chemical base of paint that is 100 % compatible with this stuff. I.e. the plastics must not become brittle from being painted (necessary for helmets, obviously, most favorable for computer cases ;-) A body shop might produce a custom mixed colour, matched to a sample (for example from a non yellowed part of the inside). The gloss could be adjusted with a transparent cover coat or lapping of the hardened paint.

In any case I would perfom some experiments before I attempted to paint a valuable case.