If you ever doubted this old adage, you need to own chickens. Just looking at egg yolks makes it clear how their diet affects them. Check this out.

I was enjoying my breakfast of hard-boiled eggs yesterday, I thought the yolk looked funny. There was a lovely orange border around the yellow center.
After briefly worrying about poison, I thought back on what the chickens had been eating a couple of weeks ago.
Shrimp. Kathleen said chickens love the shells, so I gave them a big pile, which did quickly disappear. I guess after they went in, they dyed the yolk, just like how flamingos turn pink!

How about that? I do notice that yolks are more pale when I have no fancy chicken feed available. And they are very golden when they get lots of veggies or can free range.
I hope your brain feels smarter now.