If you want your "upper jaw species" to develop moreGreen pattern, then go for "Black" or "Chocolate brown" substrates. "Red" and "Tan" also helps but not overly green. However "Black" substrates makes lower jaw bichirs to turn very dark and makes the patterns and base color to almost blend. Making them to look like have a burnt color. Mix river substrates also helps develop Greenish yellow patterns to most upper jaw species. For lower jaw, the best substrates color are Tan, Brown, Red, Orange and Pink to get more contrast on the patterns. It also prevent the patterns and base color to blend together. As for the white substrates, it will only bleach out the color and it will not show the true color/patterns potential of any bichirs.
The hardiest bichir to make it's patterns show is the Polypterus bichir lapradei, especially captive breeds. I noticed that most captive breed can easily change it patterns or washed out, compare to wild caught that it can sometimes maintain the patterns.
I prefer the best substrates for my bichirs is Tan, Brown or Red.
I hates white substrates so much as it bleaches out my bichirs, not just bichirs but most of my fish.
I never try blue, green, purple and yellow so I got no comment on those colors.