If you can let them freerange where they can snack on the grass and other vegetation you'll get orange yolks. I've heard cracked corn will do it too, but it's not good for them to eat too much of it since it's low in protien etc.
At the times that I am not able to free range my chickens they always have plenty of
cracked corn and layena (or purina layer crumbles) and they yolk gradually goes from
the deep orange to a lighter color.
Free range for the oranger colored yolks
As far as I know marigolds are edible for humans, so chickens should be OK. I did try feeding mine some once a few years ago, but they didn't go for them. I think you can give it a try.
Marigolds will improve the color of egg yolks and one national seller of chicken, Perdue, advertises the claim that their chicken tastes and lookes better due to being fed marigolds.
The old guy down the road used to pick Marigold petals. Boil them in water and use that water for his chicken porridge. He fed that once a week.
His egg yolks were always a dark orange.
I know one lady who juices feed carrots and uses that juice for her chicken porridge. Her yolks are dark too.
I have heard of people using Calendula petals too.