Yolk color is a function of xanthophyll in the diet. Backyard chickens tend to get food scraps that contain more xanthophyll than there is in commercial feed, which is why backyard eggs yolks tend to be more orange. If you want more orange in your yolks, you can feed your chickens stuff like marigolds or cooked carrots.

The color of the yolk depends on what the chickens eat; it doesn't really indicate anything about the nutritional content of the eggs. Supermarket "organic" eggs that have a dark yellow yolk might just mean that their chickens' diet included pigments that caused that. A true free-range chicken that had a more wild diet including insects and seeds other than wheat or corn will also tend to have eggs with a darker yolk.

>...though I don't know what even plays a role determining that.

Age is definitely one big factor. The longer they sit the lighter they get. But I suspect diet also plays a big role, and to a lesser extent, lifestyle/overall health

Oddly enough you don't want your eggs too fresh. I remember my grandmother saying that you can to keep them for a couple of days before eating - something about the texture of the white