The color laid down on the outside of the shell in the final 30 minutes before the egg appears is intended by maw nature to protect the embryo from the radiation of the Sun's rays until such time as the hen covers her eggs and begins incubation. Other than that (plus the increased cost of feeding most brown egg layers) the eggs are identical. In fact the brown color isn't even skin deep because the interior of all brown eggs is white. However in the final month or so of a hens laying cycle she will run out of pigment and all her eggs may will be cream or even pure white anyway. Good luck trying to convince your customers who may be 200 or more years removed from the land that this fact is true.
A good lessen on evolution can be learned by building 10 or more Blue Bird nest boxes. The Eastern Blue Bird almost exclusively use to lay robin blue eggs. As Eastern Blue Birds laid in more and more manmade structures, blue birds began to lay more and more pure white eggs. The nest boxes didn't cause this, its just that more and more blue bird chicks survived when humans became involved in their welfare. This has resulted in more white eggs hatching and surviving to adulthood, hence more white blue bird eggs. Like begets like. It always has, and it always will.