Goldfish losing color after changing food?

ok so i have 4 comet goldfish one is completely white and the others are white and orange but now the other 3 are almost completely white and i cant tell them apart! i changed their food from tetra color to wardly Goldfish flakes cuz i thought it would be better for them cuz it says goldfish on the container and the container doesn't say anything about enhancing the color and before i changed food their color was fine nothing wrong with them and i changed their food about a couple months ago now and i noticed their color is going away and i had them since for like 5 months when they were really small and their the only fish in my tank and i change their water when i change the filter and i have this stuff you put in the tank when you change or add water to remove the chemicals and stuff that is not good for them they dont seem to be stressed or sick their always swimming around happily so could it be the food?