Im not sure, it's strange as its only just started happening.
Prerhaps you could let some natural light through instead of the aquarium light - open a window if you have one near - it won't be as bright and you could try to gradually introduce him to light again. You could always ask at your fish store or try a site specifically for sharks. I hope all goes well for you
Meaning my sand/gravel? I have a light brown/tan sand. The normal color
Okay don't quote me on this later but I have a theory that certain fish try to match their colour to the substrate in order to somewhat camouflage themselves. For example light colored substrate light fish dark substrate dark fish. And when the lights turn off its dark so they get darker. And so on when the lights come on. I have light colored gravel in my 55 and when the lights have been on all day my rainbows and leopard gourami are much duller than when the lights come on in the morning.