Black substrates/bottom will only make it look like it's been rolled over some charcoal. That's why I don't really like black substrates. Brown can have the same effects, but with dark tan or red it will turn the patterns to florescent green and have a balance base color of golden brown, tan yellow, greyish green or olive or any of this mixes. I also tried blue as Fishn'Cash suggested before and I don't like the result. It made my ansorgii pale, it looses the florescent green patterns of my laps and turned it to dark brown and paled out my CB endlis. Some of my newly introduced bichirs don't even color up.
As far as my experment of tuning up my bichirs colors, I like dark tan and red.
Right now my 375g tank have total of 155 lbs of sand which have mix of 70 lbs of red, 40 lbs tan, 25 lbs brown and 20 lbs of black.
Now my substrates looks like this. (just a preview not full tank shot yet as some of my new bichirs are skinny and pale)