If you grow marigolds and also have a chicken flock, harvest and dry the marigold flower heads and mix the petals with the hens' scratch. The yellow of their egg yolks will be an even more intense golden color thanks to the natural orange pigments in the marigolds. Some chickens even consider the petals a treat.
This tip is taken from a National Geographic special issue on color, which mentioned that some European countries regularly add marigold petals to their chickens' feed.
While this tip will give you brighter-colored egg yolks, the marigolds won't make up for the natural diet of weeds and insects that give eggs from free-range chickens more color and more nutrients. Free-range eggs have more vitamin E, beta carotene and omega-3 fatty acids than eggs from birds confined to houses.