I could be wrong, but I've noticed over the years that as juveniles - the substrate will have a much more noticeable affect on the fish. But as they mature into adults, or reach sexual maturity rather, their color is constant and the same regardless of substrate color and décor. At that point, the only thing affecting a mature American cichlid's color would be breeding or stress (IME)My guy showed different color but I went from pure white sand to a dark mixed gravel.
I wish my GT was bigger now Damn they grow so freakin slow!