Yellowness of egg yolk

The color of the yolk is based on the chicken's diet. I eat vegetarian fed eggs from the grocery store and they have deep yellow yolks. If I go back to buying standard white eggs its a bit disconcerting because they have very pale yolks.

In the fall is when the eggs are the orangest for pasture raised eggs, again something about what is available to feed. The only way I can find these eggs is to get them from the farmers market or from a coworker who raises hens themselves.

Regarding the last question:

Do egg producers and/or chefs use dirty tricks to effect a yellow yolk? Dye? Food additives?

Yes, they do!

In fact scientists have experimented with food additives in order to control the color of the yolk. Interestingly enough the preferred color of the egg yolk differ between countries and even between regions within countries.

One study examined the red and yellow additives from marigold flowers and paprika fruit. Note that artificial coloring additives are banned in the US, thus the interest for natural colorings.

Details can be found in: Egg Yolk Color as Affected by Saponi?cation of Different Natural Pigmenting Sources

My local farmer tells me that the color of the yolk (and the color of the chicken fat) is directly related to how much fresh grass they get in their diet. They scatter the feed on the ground and the chickens pick up some grass as they eat. The chlorophyl in the grass is the specific thing that affects color.

If there has been a drought, causing the grass to brown, or if it is winter and the chickens go outside less, the eggs (and fat) become paler. So, the color of the yolk suggests whether the chickens are in a real free-range environment, which also affects flavor.

I think diet is the most important factor, but the breed of the hen may have an effect as well. We buy eggs from a local farmer, and a carton will have blue, green, white and multiple shades of brown and tan eggs. The blue and green eggs almost always have more yellow/orange yolks. And this was the case with our previous farmer as well.