From my experience, they don't color back up. Once it's adjusted, new growth will be purple. If they are shedding repeatedly, they are usually stressed by something besides light. They lose color when they shed too many layers too fast. The initial reason for shedding is to get rid of algae that might be growing on its skin so this might be a once-a-month thing. If it's shedding repeatedly, something is probably irritating it. Could be pH, nitrates, phosphate, or some other parameter out of wack. I had 2 purple blades in a tank that was a disaster. They shedded probably twice a week until they were gray before I got the tank under control. Phosphate was the problem. They were healthy for about 5 months before I had to tear the tank down. They stayed gray except for their tips where they had new growth. I could be wrong but I thought I'd share my experience. Too many people give "theory" answers on these forums.