High nitrates shouldn't cause a loss in color...unless you mean they're just turning brown. If they're turning brown, I wouldn't feed...means there are plenty of healthy zooxanthallae.
If your scoly's are turning white/pale, then yes, you most definitely need to feed. I use a homemade mixture of blended raw shrimp, scallops, oysters, clams, seaweed, garlic, etc. But, feeding will only help keep them healthy...not fix whatever issue you had.
Loss of color usually results from high-intensity lighting, zero lighting, or a sudden change in water parameters (too much GFO at once, huge water change, etc.) I would figure out why they're bleaching (if that's what you meant) before hoping just feeding will help.