Get some pvc pipes that only they can fit in and try to place the food in them. Might help to avoid the other gluttons in ur tank. The graying out happens with these guys normally. It can from stress to just plain old feeding time. Some say it's a dominance issue as well but can't completely go with that as it'll happen at other times.
Yeah, they need places to hide to be happy... But they will change colors pretty often depending on mood and their interactions with other fish/loaches. The biggest (head honcho of the shoal) loach in your tank will normally stay somewhat discolored. You can also feed them cooked mussels, I can't imagine some of it wouldn't fall down there for them. They are norm like $2 a bag, pre-cooked in the frozen section of the seafood... just rinse/defrost under warm water and just pull out the mussel from the shell and toss it in there. Raw/Peeled shrimp is also useful, but a tad more expensive than mussels.