3 months and still seeing nitrites. Any nitrites will effect the fish, stress them out and cause them to be dull in color and not look as bright as they once did. Sorry but regardless of what they are fed, nitrites will slowly them and has been for a while.
Once the nitrites are gone for good, quality flake food mixed with fresh fruit and blanched vegetables and live or frozen food once in a while will bring out their color. Until the nitrites are gone not even the most expensive food nor the best lighting will help with their true colors.
I am concerned about you still having nitrites at all after 3 months. The cycle should be done. Did you cycle with these fish from the beginning? Have you been using dechlorinator/water conditioner with every water change? Have you been vaccuming the gravel when you do water changes?
Some ore specific details on your tank might help us figure out why things are not going quite "right". Ornaments, filter, substrate, And your actuall readings for nitrites, nitrates, phosphates, Ammonia and pH could help also. Aslo, did you add or are you using any chemicals to alter the tank parameters? What do you have for media in the filter, i.e. cartidges, sponges, biofloss, etc and how often do you replace them?
Don't get discouraged, it happens to the best of us. But the nitrites are the cause and nothing is gonna help the color of the fish until we can figure out how to eliminate them. Not losing a fish is a good thing and the water changes are probably the reason so you have done good. Now let us help you get the rest taken of so you can concentrate on enjoying your fish instead of worrying about them.