Fossorochromis are not a fish that males color up early or permanently. Large males may color up in a mixed tank, but males often may show no color when around more dominant males. A male colors up best when in a group of other Fosso.
What other fish are in the tank? What other males are in there? Suppose the fish could be male, but hard to guess for sure.
Thanks for your reply noki! The tank is supposed to be all males. I have a lethrinops mbasi, fire hap, mdoka white lips, regal peacock, compressiceps, VC-10, blue ahli, tangerine tiger, fireline mloto, mylochromis ericotaenia, yellow peacock, dragon blood, red top lwanda, albino ruby red peacock, otter point and the sand diver. He/she is not the smallest fish in the tank at about 4.5", but only the ahli is smaller than it is. (The others go up to 8" or so.) None of the others pick on it that I ever see. I do see it developing some yellow on the tips of it's fins and on it's lips, tail and some gold down it's back and on it's head, which the picture does not show. I guess he needs more time. Do you think a grow out tank would help?
Remember that in an all male tank there is a tank boss and there is a fish at the low end of the pecking order. The low end may not color well ever.
A fish might show poor color in the main tank, color up in the isolation tank and drop color again when he is back is the same situation in the main tank.
I see other fish on your list (i.e. lethrinops) that often do not color well in all male...so your sand diver may move up the pecking order as he matures. That is why is takes a couple of years to achieve stability in an all male tank.
Thanks for the reply DJRansome. When I look at my tank, the statement you made about it taking a couple years for an all male tank to stabilize makes sense. This is especially true when you have fish of varying ages and physical statures always changing. Once all are mature, then the tank will be stable and I will know who is who.