i just aquired bunch of these lately. 14 mega and 3 indos. I noticed before they settled, via transport in bags and when first went in tank, they where more white and pale. After settling, the fish had a dark hue on the topside and fins darkened. 12 of the megas died due to bacterial infections. (I knew some would, that's why I bought so many.) What I noticed which has relavance to your question... the ones that died, the fins would clamp, tail drooped and become wobbley, and as well the coloration would pale.
I think the color has to do with their overall well being/stress levels. Darker, the better. Have seen healthy adult fish, and they turn almost black topside, like carbon chrome. Beauties.