Why do flamingos change colour when they eat?

Well, their color is the result of their diet. It's all about the algae! But, they usually stick to one type of algae so they very rarely change colors, but it'd be because they eat two different types of algae. Let's say it's eating green algae, and after a while it goes on to pink(ish). It would have green feathers that change to that color. I hope that answered your question. :)