Not allowed. Any color other than white traps the suns rays, heating up the internal structure. Which is made of composite. Which loses its strength very fast above a certain temperature.
I'm about to take up gliding and the only two things that I realy fear are a winching incident or a mid-air collision. Following a collision in the mid 90s that claimed a cadet and an instructor, the ATC have had large dayglo orange stripes around the wings of their Vikings, so I wonder why they have them and civvie gliders don't? Though your reasons for not having them make perfect sense.