I think Fritz could be the purple guy, but also could NOT be phone guy. This is because his pink slip says "first day on the job? seriously?!" and phone guy obviously has been working at the place for more than a single day.
...and yeah, um... it's pretty obvious why Fritz could be the purple guy, I mean, he tampered with the animatronics and all, and purple guy would have motives to do that.
Also, Mike could ALSO be the purple guy. Think about it, Fritz gets fired, but changes his name and gets a new job at their new place. Then, on night 7, he tampers with them again, causing god-knows-what, for god-knows-what reason. Thus explaining the concept of what we know only as the "Custom Night". It makes sense.