Dark substrate will definitely bring out the colors. I used black sand in the past for my firemouth and convict, and the difference in color was like night/day. I ended up going back to natural colored sand/stones because the problem with black substrate is everything pops out, including fish poo.
How big is the tank? You can try adding a tankmate. When my convict roams my tank and runs into my JD she'll flare up like crazy. Dark black stripes on solid bright white, with green/blues shimmering on her dorsal and tail fins. Never thought convicts could look that nice.
Also, you just got him, so give him time. He's probably a bit stressed.