Oxidation maybe? It would probably lead to color loss as carotenoids age. Would cause the yolks to eventually fade. I just googled -- eggshells increase in per

Oxidation maybe? It would probably lead to color loss as carotenoids age. Would cause the yolks to eventually fade. I just googled -- eggshells increase in permeability to oxygen after the first week, so my guess is this is what you are seeing!