I attended a school which set a different color notebook for each subject which made it easier to keep track of one's subjects especially when moving books from classroom to classroom. I still use this method in my adult life as it keeps me better organized (on paper)!
The book was used to document items that were in an overstuffed house. Yes! I mean the owner of the house is a horder. This was one way to reconcile getting rid of unnecessary papers and documents.