Are there color enhancers in the diet?
Look for terms like Spirulina, Bio-Red, BetaCarotene, Canthaxanthin, Marigold petals, Xanthins, Shrimp Oil, Synthetic and Non Synthetic Carotenoids, Color Enhancers…On the label.
Generally, the shrimp oil is the most expensive. It performs as well or better than the synthetic carotenoids but either is acceptable.
Spirulina cannot push color unless the fish are exposed to sunlight. None of these color enhancers are hazardous to fish but can make a fish with a yellow head YELLOWER and so they say: a fish with a tendency towards pink pinker.
Without sunlight, Koi will not develop the depth of red that folks find desirable. Conversely, in water that is VERY warm with too much sun, fish will get brassy.
No color enhancer can replace the irrefutable contribution of genetics and sunlight to color.
The Dainichi food linked above is a “win” because it has a superior formulation, excellent color enhancers, and best of all it’s got large AND small pellets to suit a variety of fish sizes.