Saiyo's color change is probably genetic. Plus, goldfish are notorious for changing colors as they age. It's probably not diet related because I have different shades of white in my koi, and they are all on the same diet. But there are some diets that do change the shades of white slightly. A high krill (I think it's krill) diet will cause white to become slightly pinky.
If you are worried about her not getting enough food, you could toss food all across the pond as Fishy suggested. I read about that being used to help less aggressive koi feed in large ponds. I do that to an extent because my large chagoi is such a piggy.