Could be a lot of things, but usually a change in diet is the primary one. Pigment is supplied by nutrition, so if she lacked a certain nutrient then her eggshell can be lighter. I can't remember off teh top of my head what colors eggshells, but another example would be the yellow color of the yolk, caused by the pigment xanthophylls, which also comes from the hen's diet. If you look at a good layer's legs and feet (ones with yellow, of course), you'll notice that they are whitish yellow, or bleached, rather than golden-yellow (characteristic to poor layers). The lack of pigment in the legs of a good layer is due to the fact that the hen contributed most of her pigment to the egg.
Oh my gosh I love that book! I have a copy for Poultry Bowl. If you look at the sex-link breeding in Raising Your Home Flock Chicken, it's incorrect, so I send an email to the organization, and they said they'd change it in the next edition. That's actually where I get a lot of my information, including xanthophylls.
Here's a sheet from the 4h avian bowl manual that I was just reading today!
Hmm...that does seem odd, usually that would result in MORE pigment. But, I suppose she gets the pigment from her feed, but she eats less of that due to the free-ranging.
