So there are two purple guys. The main purple guy that we see in most games is William Afton, he is both the phone guy and the guy who murdered the children, and as far as we know, there is no specific reason why he is purple, he just it.

On the other hand, in Sister Location, there's a series of minigames showing someone gradually turning purple and decrepit. That's a different purple guy, and there's a reason for that! It's actually Michael Afton, Willian Afton's son, and the reason why he turns purple is because he died and his body was taken over by animatronics (Ennard). So his skin turns purple either because it is de-compositing, or because of the animatronics inside of him.

I hope this answers your question!