Most fish can change color to some degree. Guppies are no exception. Their coloration depends on factors like lighting, backgrounds and diet. Foods rich in certain pigments help keep guppies healthy and vibrant. Feeding a variety of nutrient-rich foods can get your guppies looking their most colorful.
Prepared foods consist of packaged, dry aquarium food. Flakes are the first that come to mind, but pellets and wafers also fall into this category. Some brands of prepared foods make the claim that they contain color-enhancing nutrients. However, fish nutrition is not fully understood, and some products may lack micronutrients fish eat in the wild that can help improve their color.
You can buy liquid supplements that contain nutrients known to enhance color and healthy. Since guppies are not picky eaters, you can feed them freeze-dried foods like freeze-dried brine shrimp and bloodworms. You can pick these up at most pet shops. You can soak freeze-dried or prepared foods in these supplements -- the manufacturer's instructions will say how long -- and then feed them to your fish. They often contain highly unsaturated fatty acids, which improve fish health and color. You can find these supplements at high-end pet shops and online. However, as with other artificial color-enhancing foods, they may lack some of the color-enhancing nutrients found in wild guppies' diets.
You can also feed your guppies frozen foods, particularly crustaceans, to enhance their coloration. Crustaceans are rich in carotenoids, pigments that contribute to red, orange and yellow coloration in guppies. You can feed guppies small frozen crustaceans, available at most pet shops, or scrape small pieces off frozen grocery seafood. You can use a cheese grater to shave off small bits of seafood for guppies. Guppies will eagerly eat crustacean meat.
Live foods come the closest to what guppies eat in the wild. It stimulates their hunting instincts and mimics their wild behavior. It's also the best way to ensure that none of the nutrients go AWOL in the freezing/freeze-drying process. To take this a step further, you can soak live foods in supplements to boost their color-enhancing nutritional value. Guppies will relish in live foods like brine shrimp, bloodworms and mosquito larvae. These foods are rich in caratenoids.