strikes me as odd!
I believe/presume the colour of the shlls is related to the diet of the hens.
I remember a time when you could buy white or brown, (I suppose that was about 35-40 years ago?), and I think there was a premium for brown... better diet perhaps?
I've been a regular visitore to USA over the years, (Michigan), and I don't think I've ever seen a brown one... they've always been white to the best of my recollection.