the secret ingredient for colouring up fish is
astaxanthin.
actually, it's not really a secret.
Flamingos get their colour from krill, many fish foods contain Spirulina, aquaculture uses salmon skin in formulated foods etc etc... I've seen FW swordtails fed so much of this stuff, you cut the fish and the flesh is bright red inside.
Try prolonged feeding using fresh macro-algae and/or make your own foods (search threads such as
this one) and add lots of salmon skin.