Food can be a source of changing color, the better the food the better the color. I'd reccomend a high quality frozen food like formula 1. Cyclopeeze is also good, I have the freeze dried kind of this which I alternate with the frozen. Also if your water parameters are bad it will cause them stress and poor health. Do you have good test kits your can give reading from? How old is your tank, how large, what do you have for filtration.