Is there a rule or unwritten rule that Professional Heel Wrestlers can or can not, or must wear certain colors?

There was a time back in the day where babyfaces and heels were very "white hat/black hat", but I think we've passed the point where we have to guess who the bad guy is by what he's wearing.  White gear can be just as obnoxious an eye sore as coming out in a pair of plain black trunks and boots, so there's not really a guide line for what heels and faces wear anymore.