I know this sound kind of silly, but it looks as if the smaller of the (2) is displaying a more defensive coloration. Its amazing how quickly their colors can change. The larger of the (2) probably doesn't feel threatened by the smaller one, so its color is more subdued.
I've noticed this with a tank of Sacramento Perch that I have the regular pleasure of observing. Of the (3) Perch in the aquariium, the largest is more of a subdued color, while the two smaller ones are always coloered a little darker. When any food is introduced to the tank, the smaller (2) will lose their dark mottling, and tend to look more like the largest. They all maintain this coloration until the slightest bit of aggression or territorial dispute begins, and then they'll switch back to their much darker mottling.
Perhaps you might add some fatheads to act as dithers, and see if anything changes. I too like the darker colors of a SMB myself. Regardless, VERY NICE FISH!