If one wears a toupee that covers approximately what a normal kippa would cover, does he also need to wear a kippa?
Does it depend on whether or not people know it's a toupee and not just his normal hair?
Mishna B'rura (2:12) writes that there are those who say that a toupee-wearer must wear a kippa on top of it because of maris ayin, but there are others who say it's not necessary.
In other words: technically, it is a valid head-covering. But since we are worried that people will think that the toupee is his real hair and that he is not wearing a head covering, some say that he must wear an additional (clearly visible) head covering above it. [See also P'ri Megadim (EE 91:4).]
This would imply that if it is obvious that it is not his real hair (e.g. a clown wearing a rainbow wig), then an additional kippa is unnecessary. For practical issues, CYLOR.