Yellowness of egg yolk
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 Saponiﬁcation of Different
Natural Pigmenting Sources