I’ve noticed that too, especially with African cichlids. I saw a video somewhere I can’t remember of a tank that had a tube attached to a skylight instead of electrical lighting and it looked amazingWow! Those are amazing! I’ve always found sunlight to bring out the best colors in any of my fish.
I know what you mean. All of my tanks are in the garage and when I open the garage door, all of my fish look much more colorfulBesides the broad spectrum of sunlight the direction matters a lot. In most cases sunlight hits the tank from an angle. Especially in some Tanganyika cichlids light from the side can make a huge difference. I know somebody who has an additional light from the side for featherfins and it makes a huge difference. You suddenly see completely new colours.