I tend to look at it from a different point of view. Some may agree, some may not. If you are piping in a solo capacity and not with a band....in my opinion, there are no rules. Just make sure you look professional and respectable. If you want to wear a colored shirt then do so. I used to wear a law enforcement shirt with no insignia what so ever on it, horse hair sporran and spats with diced hose tops when playing for the Park Police graduations...then for my nightly gig would wear several different outfits ranging from contemporary to historically based. Mix it up a bit. Just make sure that whatever you wear goes with the event you are playing at (Thats where the professionalism comes into play).
Remember...."Rules are meant to be briken..." (try not to throw to hard when launching projectiles at me!)
Cheers,
CHad