my kuhlis change colour all the time. i try not to worry about it if they are acting normal. for instance, if i'm cleaning in the tank and i move an ornament, all the kuhlis underneath it will normally be very pale, but they will colour up later on once they are out for a while. since you have sand for the subterate, i doubt that is making them pale as it is the most natural subterate around. if they are consistantly pale i'd check the water conditions. if they are good then just relax. some of mine will pale out quickly when frightened by an noise and some stay bright almost all the time. it's not that big of a deal with this species.