An NPR story I found pretty fascinating.
[T]he reason Americans eggs tend to have bright yellow yolks has nothing to do with “hormones” but rather with what we feed our hens. Beyer says egg yolk color is almost entirely influenced by the birds’ diet.
So if you’re feeding birds yellow corn, “it gets in the egg,” he explains. “But if you had a situation where you’re feeding birds white corn, then the egg yolk could be white.”
In most parts of the world, he says, diners prefer their yolks with a sunnier disposition […] On the other end of the rainbow, says Beyer, are the yolks in some parts of South America, where hens will peck at dark red annatto seeds. The result? Brilliant yolks ranging from dark orange to red orange to pink, Beyer says.
Many egg eaters assume that darker yolks are a sign of higher nutritional value, but both Beyer and Jianlin independently told me that’s not the case at all. Although chicken feed does influence the nutritional value of birds and their eggs, the researchers say yolk color won’t tell you anything.