For some reason one of my Jersey Giants laid a white-shelled egg. Previously all the eggs have been a shade of brown with one gal laying a pinkish colored egg. 2 days ago, my 16 gals laid 13 eggs and one was white. Yesterday all were the normal shade and I'm wondering what I'll get today. Yes, these year-old hens are hatchery stock so I don't expect perfect eggs, but the white one surprised me. All hens are acting normal, and the egg itself was perfectly normal-nice strong shell, tough inner membrane, and a beautiful yellow yolk. I was simply surprised to get a white egg after all the brown eggs. The diet has not changed: Purina laying pellets, some scratch feed, and daily fresh greens, and bread as a treat.
How the color gets on the egg is fascinating. Think of the reproductive tract as an assembly line - sometimes there are glitches. However, egg color can also be altered by disease, so do keep a close eye on your flock. Here's a post I wrote about egg color and how it's created: http://www.hencam.com/henblog/2012/02/brown-green-blue-white-chicken-egg-color-the-real-story/
Had that happen to us once, got a beautiful white egg from the sex-links, they were healthy and nothing much had changed. Just a one off thing, I suppose. We took photos, but I can't remember where they are.
If it's consistent, there might be a problem, but I usually don't worry if it's just an occasional thing.