To celebrate the game's first birthday, Niantic has announced the game's first Anniversary Event. While it's surprisingly limited in scope, this may be due
Ash Hat, Galaxy S7, Samsung Galaxy S, Pikachu, Olaf, Note 7, Emojis, Phone Cases, The Emoji
Person C knows his had is black, because if it was white, Person D would have seen two white hats and said his color ;D The best riddle I've ever heard. (I'll post answer in comment section if everyone gives up).