They have to be a very light colour to prevent UV damage to the composites. There's no rule to stop them being bright yellow (for example), but it probably wouldn't help much.
Contrary to what many glider pilots may think mind you, there's no rule to prevent them putting a small air generator and some strobes on. Equally, there's no prohibition on their flying at a safe distance from other gliders, on flying normal joins and circuits at their airfields, or other innovations that would stop them scaring the rest of us from time to time.
G