Google has failed me (or I, it) in locating said books. (My search was: “High school English textbooks, 1970s, hardback, red, yellow, green”) These English textbooks were rather small (maybe four inches by six, but a couple inches thick), hardbacks, and came in a different color for each grade (9-12). Three colors I remember were yellow, red and green. The other may have been blue; it may not have been. I can’t remember.