i feed mine hikari gold and his colors are very vivid.
switch that up with other things to supplement what the pellets lack.
veggies, krill, shrimp, nightcrawlers, fruit are good choices for treats.
if you are really concerned with his coloring the best you can do it keep his water parameters in the best shape possible.
ideal water quality and a good diet allow for the best coloration and growth you can provide.
