Below is a link to a discussion on the topic. You want to limit red enhancing foods including krill. There is a name for the red enhancing ingredient. I'll have to look up the spelling, something like astaxanthin.
http://forum.simplydiscus.com/showth...or-yellow-fish
Below is a quote from another link which I 'll post below the quote:
Natural sources of pigments are available in the diets of most fish. Color enhancing diets may contain additional natural pigments to enhance colors of ornamental fishes. The carotenoid pigment found in most marine and a few freshwater invertebrates is astaxanthin. This pigment gives the characteristic color to the flesh of salmon and is available in the diet of aquarium fish in shrimp and krill meals and salmon (fish) meal used as sources of protein in some feeds. Pure astaxanthin or canthaxanthin (synthetic astaxanthin) may also be added to fish feed to enhance red and orange coloration. These carotenoid pigments are often added to feeds for farm raised salmon and trout to give fillets a desirable red color. Xanthophylls (yellow pigments) are found in corn gluten meal and dried egg that may be added to the diet to enhance yellows. The ground petals of marigold flowers have also been used as a source of xanthophylls. The blue-green algae spirulina is a rich source of phycocyanin and may be added to a diet to enhance blue coloration. The expense of supplementary pigments often limits the amount used in tropical fish feeds.
http://www.bigskycichlids.com/coloration_article.htm
Hope that helps. Most foods contain ingredients that enhance reds. I have both red and yellow fish, so I try to minimize the red enhancer, but have never tried to enhance the yellow.