If it's lutein that makes the most difference, then tomatoes should work as well.
I think there are many sources of lutein that would darken yolks - spinach, alfalfa, tomatoes, etc.
"Marigold (Tagetes erecta L., Asteraceae) is not only grown as an ornamental, cut flower, and landscape plant, but also as a source of pigment for poultry feed. The pigment is added to intensify the yellow color of egg yolks and broiler skin. It is composed of esters of xanthophyll (lutein). Finely ground blossom meal, often enriched with an extract, or the extract itself, usually saponified for better absorption, is added to the feed. Marigolds are grown for this purpose in various locations in the western hemisphere, primarily in Mexico and Peru, by and for various companies who produce feed additives."Horticulture, Purdue University
Without processing, I can't see why a chicken would eat a Tagetes flower. However with tomatoes, alfalfa, spinach, etc. - we know that they will eat them and benefit.