I dont know how it affects coloring but I always fast my fish once or twice a week. I think the fish look healthier the fish fed constantly. Thats my opinion and in no way fact. I do know that fasting does allow time for your filtration to catch up biologically and the water looks extra clear when this is done. I do two to three 20% water changes a week but the day after fasting is when I think my fish look the best. They do get a lot more aggressive though being that they are hungry, so fins start to look tattered on occasions.