A lot of people recommend leafy greens, like kale, spinach, collards, mustard etc. The grass clippings are good too.
Here's some further info:
Yellow to orange yolks
Seaweed meal (algae),
dehydrated alfalfa meal, corn
gluten meal, flower petal meal,
dried chili peppers, powdered
African red peppers, dried sweet
potatoes, dried carrots, corn oil
products, food grade fat soluble
dyes, etc.
Feed recommended levels of xanthophyll bearing
materials for desired egg yolk color.
Yellow =13 mg of xanthophyll per lb of feed
Medium orange = 23 mg of xanthophyll per lb of feed
Orange = 34 mg of xanthophyll per lb of feed.
Maximum color will be present 10 days after the
hens are placed on feeds for yolk color.
Imp