He is dark from stress. Give it some time. Keeping fish all your life you should have an idea about these things.
As far as getting your fish used to movement, you need to be around the tank all the time.
My GT's tank is center of all the action and he is bold and beautiful. Badass glassbanger too. Tanks that rarely see action will have shy fish.
Yes I would generally attribute the darker colour to stress. However I've read on this forum that a black background and black sand will prevent most FH from displaying their best colours. As mine has a dark/black base colour anyway, that will certainly be the case.