Consensus is that the trim is actually stainless steel, and the discoloration is oxidation. It seems like Meguier's Ultimate Compound is a good cleaner for the piece, but it should polish out.
Meg's cleaner wax is also recommended. You may have to rub vigorously. Failing that, many have been replaced under warranty. This is not simply a single discoloration cause; because some pieces won't clean up, and others clean easily with no effect to the finish.