I don't think there is anything worrisome here. The fish looks good. It's on the dark side and that may be because of the substrate, as stated above. In fact, IMHO, it is by far and large the most likely cause. No need to invent other explanations.
The younger fish may not have the ability to blend in as well as the bigger fish simply because of the age. Or could be the gender difference. Or that it spends more time in hiding during day hours.
Black substrate brings out colors in non-benthic fish, I'd think. Benthic fish, that is the ones that live on the bottom, will invariably try to blend in with the bottom, unless/until a breeding season, territorial disputes, what not, that requires visibility.
Bullying stress usually causes paleness, the opposite of what we observe here.
It is possible that your specimen is melanistic = a mutant that has excess of pigment, the opposite of leucism, albinism, etc. It's very rare but happens. This usually waxes and wanes, comes and goes, as in piebald fish too.
