Garnet will bring more of the patterns/base color out. If you want the greens out of a retropinnis I would go with black sand. On garnet they still get green, but IME nothing compared to black. Keep in mind though it can vary from fish to fish. The retro in my pic was decently green on garnet while it was more of a dark blue/black on black sand. I also have 4 retros now that are a more lightish blue-green on garnet. So it can vary from fish to fish, but I would say black draws the greens out a lot better.