1. "Nothing".
2. "Your imagination".
3. "Time". But you mistold the riddle. The normal ''I turn polar bears white' riddle doesn't end with "What am I?", it ends with "Can you guess the riddle?". That's the version I know. That's why people are responding "No", because the original answer is "No", since riddles cannot be guessed, only solved/answered. Otherwise, "Time" is the only true answer I am aware of. Unless you wanted to say something like "Non-existent".