yup agreed, its work, i think he is more comfortable of the black gravel thats why he coloring up...
This is just my own opinion, just tell if this is possible or impossible...
i am thinking that FH is a fresh water fish and their color is going nice or come-out defend to their sympathies, then in order to see their true color we should make them comfortabe, right? most of the gravel in the river or fresh water is black and the white gravel is from the sea, i think they are more comfortable white the black gravel..