Question

What effect does food colour have on the amount of food fish eat

Answer 1

Absolutely none. Fish take food into their mouths and taste it. If they like it, they swallow, if not they will spit it out. The color of the flakes has no bearing on what the fish takes in and tastes in my experience.
I hope this helps : )

Answer 2

None, like they said fish take food into their mouth and taste it