It's just the fish. I have a healthy littel one which is amost half black and half white. If I would have not seen his stripes at teh IFS I may think he's another type of Dat. I agree that some of it is stress, but it atleast for mine speak more about their mood than anything else. My big one is dull when he sits on the bottom, but when he's up and swiming he has more contrast. If I drop feeder fish in he gets excited and he gets awesome coloration. These fish in my experience have one of the fastest most incredible "active" coloration of any fish I have. I can simply look at their color and know how they feel. I know that sounds crazey but it's true.
Here's a trick, try this. Do a water change, maybe about 30% mine love it and for the rest of the day they get excited and swim all over and get really bright. The next day they are dull, I can check the water and it's fine. They just liked the fresh mix, kinda like food. I say give those guys some good excitment and they'll brighten up..... Or put a bigger fish in their and they will go on the defensive (stressed mode) and brighten up.
Think of them like people. Mad/Happy makes them bright. Depressed, bored, tired, etc makes them dull.
