Not so long ago I tried making an egg based recipe from a Japanese site. It generally went alright. However, when I was comparing images of the dish from the Japanese site with what I actually made, I noticed something interesting. The color of the egg yolks is quite different!
Links to some example pages with images of egg yolks:
The Japanese eggs are noticebly darker in my opinion. According to a color picker tool, the Japanese eggs are #ff9733 and the American eggs are #f6bc00. It seems that Japanese eggs are closer to orange in color (I'm color blind though so take my words with a grain of salt).
I would have thought that egg color is generally constant worldwide, but it turns out I was wrong. After some research, this is apparently a well known phenomenon with a number of questions on Chiebukuro and other Q&A websites.
According to an interview in 東洋経済, the reason for this discrepancy is:
河岸: In the past, the main ingredient in chicken feed was grass so the color of the egg yolk was darker. But since the majority of consumers mistakenly believe that "darker egg yolks are more nutritious and tasty", we feed the chickens paprika and force the egg yolks to become darker.
N君：When you say paprika, do you mean the spice paprika?
河岸：Correct. We mix that into the chicken feed and force them to eat it. Because corn is now the mainstream chickenfeed, egg yolks have become whiter in color. That's why we add paprika and make the color darker.
The more surprising thing honestly is that American consumers didn't have a similar preference for darker egg yolks. It seems like it is sometimes a thing , but it's certainly not very common. I don't have any recollection of seeing such darker egg yolks when buying eggs.
Fun fact: Apparently depending on what you feed the chickens and the timing between changing the feed, you can even make striped egg yolks!
Any error corrections or comments can be made by sending me a pull request.