What you feed your fish is just as important as keeping the world they live in clean and healthy if you want large, healthy fish.
The first five years of a fish's life is it's growth years. During this time the need for higher levels of protein are greatest. Feed young fish food that contains 42% or more protein. It may be more difficult to find but it will help them grow more.
It is important to buy the correct sized food. A small fry cannot eat an 8mm food pellet and may die of hunger even though there is plenty of food. Likewise, an adult 20 inch koi may starve eating a 1.5mm pellet as it spends more energy trying to eat it than it receives from such a small portion size. The pellet should be a size the fish can comfortably get in it's mouth. That means if you have small fry and adult fish you are going to need to put two different sizes of pellets in at feeding time.
Feed your fish no more than they will eat in a 10 minute period of time multiple (at least 2-3 times) each day once the water temperature rises and stays above 45° F. More frequent smaller feedings are far better than fewer larger feedings. Remove the excess food immediately. Uneaten food will break down into chemicals which will feed the algae and are harmful to the fish and the water ecosystem.
If you want to intensify the colors of the fish use a food that contains carotenoids or spirulina known as color enhancers. If your pond is in full shade these enhancers will not make the colors richer. Color enhancers work primarily on reds and can make whites slightly pink.
Fish enjoy a variety in their diet too. Feed them a quarter of an orange, watermelon, peas, krill. In the fall, feed them Cheerios.
What do I do feed the fish in the wintertime? Take a break. Both koi and goldfish go into a hibernation-like state once the temperature drops below 45°F (about first of November). Do not feed them; fish cannot digest food below this temperature and it can kill them. They will be living off their stored fat during this time. Do not let the pond freeze over or fish will die. Sometime in April the water temperature will rise above 45°F and you will need to start feeding them again.