After eating a particularly orangey-red egg yolk while traveling, I remember reading egg companies could buy feed according to what color they wanted the yolk to be. Can’t find the source right now, but I found this and this.
"Consumers worldwide are increasingly aware of the importance of healthy foods. They know that a golden yolk is the sign of a fresh egg from a healthy hen. The right husbandry, welfare and diet, in combination with CAROPHYLL® carotenoids, deliver eye-catching egg yolks that inspire consumer confidence around the globe.
We all have our own notion of the ‘right’ color for an egg yolk. This varies around the world, depending on geographical location, cultural context and local traditions. It is also influenced by the culinary use for which the eggs are intended. However, consumers worldwide recognize that a golden yolk invariably comes from a healthy hen. And they are asking for yolks with a deeper shade of orange. "
Another take on it
“Does David’s Pasture Use Coloring Agents in Their Hens’ Feed?
We do not use any coloring agents in our hens’ feed, and because of this, the color of our egg yolks changes seasonally.”
