From what I know, it's not uncommon for fbts to have some color differences. In my tank of four about three of them are really bright, but one is definitely darker(not black like others have mentioned), but his belly is very bright and he's one active dude; he will lunge from across the tank when I get out the feeding tongs. While I am far from an expert, I think that behavior, eating, and belly color might be more important. My little girl that had an impaction was very bright green, but her belly was faded and she was super lethargic/would not eat. Sadly, she passed, but IME those things are more concerning than just her back color. From what I've heard ftbs can even come in a brown shade, so who knows?