That's a pretty common thing with silver and I'm not sure it's a defect. My 98' Grand Prix used to look like the bumper was a completely different shade of silver depending on what angle you looked at it.
Does anyone really care? It's not like it makes the ZR-1 any less prone to making my peepee straighten up.
The cool thing between ZR1 and GTR humpers is to make fun of every flaw.