I remember that the book "The Neverending Story" also employed a similar scheme: Red and green ink was used to distinguish between the book's two worlds. See,

I remember that the book "The Neverending Story" also employed a similar scheme: Red and green ink was used to distinguish between the book's two worlds. See, for example, https://greaterthanknowledge.wordpress.com/2010/03/01/an-inside-look-at-%E2%80%9Cthe-neverending-story%E2%80%9D/ https://greaterthanknowledge.wordpress.com/2010/03/01/an-ins... The book was published in 1979. It would be fun to hear of similar techniques used even earlier.