No limits on wheels. I've seen car go through judging with nice wheels... the judges honestly don't even appear to notice them. They know they're "free", and they don't give you crap for them any more than they would give you crap for having a really nice roll cage.
That said, they DO affect the overall appearance of the car and EVERYONE ELSE will notice. I've seen a lot of teams go through tech and judging with stock wheels, and then change them for the race. Which is perfectly legal.
I'd say that if purple wheels fit your theme and you don't want to paint your "nice" wheels, use some cheap stockers for your theme, and use the good wheels for the race. But, any way you slice it, wheels are considered safety and nobody really cares. (not to say we won't comment on the fact that your wheels are worth more than the car, of course)
Lemons South 2008 - Fail, Lemons South Spring 2009 - Fail, Lemons Detroit(ish) 2008 - Fail, Lemons South Fall 2009 - Fail, Lamest Day 2009 - Fail, Miami 2010 (Chump) - 2nd!, Sebring 2010 (Chump) - Fail, Cuba 2010 - Crew Chief, Roebling 2011 (Chump) - 8th!, Sebring 2011(Chump) - 19th!