white egg yolks

A diet high in greens and grass will give deep yellow yolks too.

Yes, my layers don't like maize (either cracked, dried, tinned or on the cob) and their yolks are a very deep yellow, which is immensly apparant if you are baking uncoloured cakes.  My hens have plenty of free access to grass and other, er, herbage as they find in the garden, but I don't purposefully feed cabbage or lettuce either.