When it comes to colours, aqua glo, although not the brightest out there, can often look the best on a fish. Especially one like a gt with that firey orange. There is a lot of red/orange spectrum in aqua glo's....looks great on a midas, or some africans like red zebra's too. It's a good all around light, fairly broad spectrum. It's what I would recommend, especially if you decide to go with a lighter substrate. When it comes to light gravel affecting behaviouir, it can happen. Usually it just affects the color of the fish, but shy cave dwellers like dempseys may be less sociable under bright lights with light gravel.