substrate will affect color only. White sand is going to make them pale and the pattern wash out. Its not permanent, so if you switch colors the pattern/color will change.
Feed a variety. Bichirs in general are not picky once they learn it is edible. Anything that can fit in their mouths is fair game to raise them on. I personally have massivore as a staple with mine. They are big, but easy to break up. I have raised 2" axolotls with massivore. Tilapia, shrimp, any pellets. they have a great sense of smell and they will find any pellets or meaty foods. Mine have learned (quite easily) to take hikari floating carnivore pellets.