The color of an egg yolk is directly influenced by the makeup of the chicken feed. Egg yolk color is generally improved with a feed containing a large component of yellow, fat-soluble pigments, such as the carotenes in dark green plant material, for example alfalfa. Although much emphasis is put onto the color of the egg yolk, it does not reliably reflect the nutritional value of an egg. For example, some of the natural pigments that produce a rich yolk color are xanthophylls without much nutritional value, rather than the carotenoids that act as provitamin A in the body. Also, a diet rich in vitamin A itself, but without A-provitamins or xanthophylls, can produce practically colourless yolks that are just as nutritious as any richly colored yolks. Since unhealthy chickens produce fewer and smaller eggs, farmers ensure that whatever the source of their feed, the quality is adequate, so there is not likely to be much difference in the nutritional quality of the eggs.
Yolks, particularly from free-range eggs, can be of a wide range of colors, ranging from nearly white, through yellow and orange to practically red, but even olive green, depending on the pigments in their food. Feeding fowls large amounts of capsicum peppers for example, tends to result in red or deep orange yolks. This has nothing to do with adding colors such as cochineal to eggs in cooking.