[Sean C]

Could be a factory paint error. Usually such errors leave off paint, though, since they are usually sprayed on with "masks". Since Shredder's brow/rim was molded flesh colored plastic there's no paint mask or spray op besides the black eyes brows. That's the odd part. Where did that paint op come from? I'm assuming that there was no hand painting, though.

A factory does this 1000 times and it's a rare variant. Do it once and you have what you might have here, a one-off accident. That's hard to confirm in loose condition or put a price on.

You'll need to ask hardcore TMNT collectors, though.