Apple likes high quality materials for their Powerbook/MBP line, though one could argue whether polycarbonate is more durable than aluminum.
The old Titanium PowerBook had a - guess what - pure titanium case and the PB/MBPs are using aluminum.
Only a small portion of the upper case / keyboard area consists of aluminum-coated plastic, since aluminum alone wouldn't be stable enough when used at that thickness. (Or something like that...)
Your local car dealer might be able to paint it for you. 😉 (Kidding)
There's is a
company selling ridiculously expensive, painted 'Books by the way.