Tell your customers that what they eat affects yolk color (true), and that the changes in color with the seasons have nothing to do with quality (any more than artificially colored salmon is of greater quality) - its simply an advertising gimmick. You've been feeding your birds food coloring. (perfectly safe food coloring).
Marigold petals are popular fo altering egg color. You can also plant (and feed) cantaloupe as treats, baked/steamed sweet potato, carrots, yams (but I said that already!) [OK, not truly, but few would recognize the difference], beets, and other flowers/veggies high in betacarotenoids.
Again, from a nutritional standpoint, the color is near meaningless. Mine free range, and during a stretch of winter, they actually turn a bit green. Perfectly healthy. Its them eating more acorns. The ducks, particularly.