Marigolds may make the yolks look yellower, but it does nothing to improve nutritional value. It's like adding molasses to your white bread dough to make it look like darker whole wheat without the benefits. The yolks of eggs from chickens fed a nutritious diet are naturally deeper in color--specifically the pigmentation comes from beta carotene contained in fresh plants and concentrating in the animals that eat them (including the bugs that the chickens est). This makes rich yolk color a useful proxy for estimating high nutrient content (especially vitamins A, D, E ). The same principles explain why the cream of the milk from cows on rich pasture is more golden. But it's important to note that it's the high vitamin A content that makes the egg bright orange, not the other way around, because there seems to be done confusion on this! Also, one major flaw with commercial feed mixes is that they supply no fresh greens or insects that ate fresh greens, so they are low in the associated nutrients--commercial feeds are great in other ways, don't get me wrong, and very convenient, but they do lack this important element. We follow a "fresh greens daily" rule, which helps keep yolks more nutritious (access to quality forage, or supplying garden and other greens--and the bugs that ate these things themselves). Fermenting feed also helps (making nutrition already in the feed more available and creating new nutrients). One goal of good farming, including backyard eggs, is not just to produce as much food as possible from a system, but to concentrate and accumulate nutrients into useful (and delicious) forms humans can benefit from--that is, to produce as much NUTRITION as possible.
