Food additive manufacturers sell farmers aditives to produce yolks with specific hues[1]. There are regional/cultural variations in color preferences, so regional farmers will target different sades.
It's slightly disingenuous to call carotenoids "additives". Although it might technically be true, carotenoids are naturally present in tons of vegetables (hence carrots) and are a good antioxidant with other known health benefits.
[1] https://www.dsm-firmenich.com/anh/products-and-services/prod...