A Touch of Grey

I had students in my US History survey class read L. Frank Baum's Wonderful Wizard of Oz (Ranjit Dighe's annotated edition, The Historian's Wizard of Oz). Discussion went well. To make sure they read the book before class, I told them they would have a little test on the book. I asked what color Dorothy's slippers were, of course, and I also asked what color Baum used at the very beginning to show the bleakness of prairie life. Most students got it right (gray), but over half spelled it "grey."

I blame it on Grey's Anatomy. What do you think?