I was in class and my professor was joking about writing notes on an entire page in one colored ink and then writing over the same page with more notes, except with another color of ink. The result would be a page of jumbled and overlapping letters and numbers in two ink colors. To decipher the notes, you would look through a color filter so that you can only see one set of notes.

A cool concept, but I’m not sure how well it would work in practice.