So, is the dom fish also dark. I can't tell from the photo.
Having said that, dark substrate, dark background, typically equates to young fish being dark. Especially in a fairly bare tank, such as yours. It's a survival instinct.
If the dom fish is also dark, and as you say all tank parameters are good, then most likely what I originally stated. Hendre posted a good example of how fish will attempt to blend in with their environment.