There are natural and artificial pigments that can be absorbed by a hen's digestive tract. The xanthylls that color yolks deep yellow to orange are the one example that we are all most familiar with. Cottonseed can produce a yucky olve green cast (as well as degrade hatchability). There are a few other plants that can affect yolk color as well, one can make them reddish, but I can't recall what it is just now. I would imagine a concentrated source such as paste food coloring could produce a really noticeable change so long as the pigments are absorbable.
As an illustration of the way a hen builds an egg it would be very useful. I don't think I'd care to do it otherwise.
.....Alan.