First you have to define what you mean by "exist." Of course every idea that can be conceived exists in some sense. That does not mean its existence is realise

First you have to define what you mean by "exist." Of course every idea that can be conceived exists in some sense. That does not mean its existence is realised or useful. Is it useful (consequential) and consistent that yellow balls exist? If yes, then they do. If no, then they don't.