It all depends on nutrients. If they're getting more, or less, eye color (and all coloration) can change. It generally takes about a year.
Example: I once got some 2-year-old pure white supposedly purebred Leghorn hens. They had whitish irises, white beaks, white claws, white legs, pure white feathering, and laid white eggs.
A year after I got them, one was creamy buff colored and the other blotched black and red, and both had changed eye color to red and yellow respectively, and their legs had gone pale yellow and dark yellow respectively, and their eggs were now brown shelled. One had black pigmentation showing in her legs too, and their beaks were now yellow, one with black streaking as well.
The only difference between their old appearance as two year olds, and their new look as three year olds, was that I put them onto a diet with kelp in it. Very nutritious stuff, that. I've seen it strongly alter the appearance of aged individuals of a few species, and in humans kelp has been proven to return white or grey hair to its youthful coloration.
Even if you're not feeding kelp directly, there are extracts in many feeds and supplements, and theoretically any very well balanced diet would do the same. There are some land plants which are the equivalent of kelp, the two most well known being hemp and nettle, but doubtless there's more I'm not overly familiar with as well.