Congrats to you on your hens! Are their beaks intact? If they are, and they get grass or grass trimmings regulary and access to bugs and room to scratch, their yolks will be as yellow as anyone could want.
My girls barely eat any layer feed in the summer they're so busy out in the garden (back yard to other Americans!) eating greens, bugs and what all, that I barely have to fill the feeders.
They only peck at the provided layer feed in the morning before I let them out, and in the evening when I'm near the coop and they're thinking about going in to roost. I'd worry about their diet, except they are thriving and their eggs are WONDERFUL.