You say you think it was supposed to be a study in contrasts, which...um? Not sure I quite understand.

Kate will correct me if I'm wrong, but I believe Mary was making a contrast between her appearance and the appearance of the other girls. Her study in contrasts essentially assumed everyone already knew her appearance because they could see her right there on the panel.

But it does invoke, at least in text as I read this summary, the idea that white is default and the other girls in her class needed describing because they weren't default.