Hmmm... I've done some checking online and there seems to be some differing opinions on this.

I was taught years ago that yolkes were yellow because of the sulphur. This was "common knowledge" at the time, but it seems—as I so well know—much of what is considered "common knowledge" isn't accurate.

I do know that sulphur is yellow because I was into geology as a kid and had a chemistry set and rock collection and everything sulphur was yellow. I just checked this online and this seems to be the consensus as well.

The opinions about the reason the yolk is yellow differ. Here are some examples.


Yolk colour varies. It is almost completely dependent upon the feed the hen eats. Birds that have access to green plants or have yellow corn or alfalfa in their feed tend to produce dark yolks, due to the higher concentration of yellow pigments (mainly carotenoids) in their diet. Since commercial laying hens are confined, lighter and more uniformly coloured yolks are being produced. Yolk colour does not affect nutritive value or cooking characteristics. Egg yolks are a rich source of vitamin A regardless of colour.

—http://www.food-info.net/uk/qa/qa-fp202.htm


Food doesn’t just have to taste good, it has to look good too: blue milk, green meat, and violet butter don’t stand a chance under our critical gaze. Everything that we eat has to look good, smell good, taste good, and have the right consistency. Especially when it comes to egg yolks, we love a golden yellow. But where does this golden yellow come from? What are carotenoids, and what role do they play in nature?

—http://www.yellow-egg.com/wEnglish/filme/index.shtml?navid=21
(Video Available)