Corn will turn them more yellow not the orange that is refered too. All your dark green leaf veggis are great for this also many weeds. Mine are eating common plantain and dandylions that grow wild here as well as clovers. If you want to grow feed for them try kale, spinach, collards ect. Many just pick weeds out of gardens and grass as long as it is not sprayed with chemicals and feed them to the chickens.
Nope, it just turns their system red for a day... If you know what I mean. Just remeber that you fed them the blasted things the day before or you'll be posting up on here in the emergency section.
Did I do this?? No... never...
I am pretty sure that the darker yolks come from a varied diet of greens, bugs and layer mash.
I do notice that whenever we keep the ladies penned up and don't let them free range in the afternoons the yolks are lighter in color for a few days and when the ladies are let out to range, the yolks turn darker.
My guess would be the insects and greens that help make darker yolks but that's just a guess.