Step 1 : Introduction to the question "What determines the color of a chicken's egg yolk?"
... Among those animals which produce one, the yolk is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Since dietary cholesterol was once thought to be the major cause of unhealthy blood cholesterol, egg yolks have been demonized and health nuts stick to eating strictly eggwhites. Now, don't get me wrong — egg whites are a great, healthy source of protein, but there is definitely room for WHOLE eggs in a healthy diet.
Step 2 : Answer to the question "What determines the color of a chicken's egg yolk?"
The hen's diet:
The hen's diet is the determining factor in the color of an egg's yolk. A hen that eats a large amount of yellow-orange plant pigments will deposit those color pigments in her egg yolks. Hens fed mashes with alfalfa meal or yellow corn lay eggs with medium yellow yolks. Those eating mostly barley or wheat produce lighter-colored yolks. Natural yellow-orange substances can be added to feed to enhance yolk color, but artificial color additives are not allowed. Source: IncredibleEgg.org