At the end of last season, I noticed that my yellow patent on the toe strap of my yellow sandals was starting to discolor. I really have no idea how this happened. It's not that something like dirt got on top of the sandal. The patent just seems to be discoloring to a blackish grey. Any ideas on how to fix this? I've tried nail polish remover but it didn't work. I'm bummed about having to give these sandals up.