Better nutrition. Means you're doing better by the chook than her previous owners, or at the very least her mother, could.
Incomplete nutrition has a knock on effect which afflicts generations, particularly through the female line as the mothers have to supply virtually all the materials the offspring are built of. Too many breeders give the hens enough for themselves and not enough for their babies as well, so offspring hatch with deficiency diseases if they hatch at all.
It takes generations of malnutrition to cause a serious impact which then takes generations of complete nutrition to erase from the family line. It can take a few generations of having them on a great diet to cause them to show their true colors, but generally even older adults will, in my experience, change color within a year to whatever they would have looked like if they'd had sufficient nutrition.
Old example now, mentioned this one a few times, but here you go in case you haven't heard: one of the most drastic color-change examples I ever had, in response to better nutrition, was in two White Leghorn hens, who were two years old, two-a-day layers when I got them. They had white skin, feathers, legs, beaks, claws, eggshells, pale irises, etc. They were either show quality or very close to it. After a year of being on the diet I give my chooks, these hens were no longer pure white, they had yellow beaks and legs and claws, with streaks of black in them, orange to red irises, black and red feathers mixed in among white and fawn feathers, and their eggshells were brown.
Once onto a better diet, the animal responds by chucking out inferior cells built on inferior nutrition and replacing them with superior cells built on superior nutrition; this often involves lessening of production or even a complete but temporary stop, a moult and a detox, but they will come out of it looking vibrantly healthy and live for longer not to mention produce superior product, whether it's offspring or eggs or meat. Their offspring will, over the generations, continue to improve on overall health and demonstrating their true type.
Just goes to show, you don't know what you're really breeding if you don't give them a properly nutritious diet. Commercial layer mash or pellets generally just does not cut it, they're mere survival rations in almost all cases, even though they're labeled 'complete'.
I've had other hens and other animals change color too, nutrition governs color in all species... A light sussex hen developed red feathers in a few places, etc, there's other examples too.
Best wishes.
I suspect it's probably more applicable to cases where pigmentation was lost due to stress, insufficient nutrition, disease, or any other cause than someone in true health reaching a very ripe age.
