The initial question I have posed was why Tibetan books (dpe cha / po ti) are sometimes painted red on the four sides. I have been trying to find some answers (even silly or wild ones) in written sources but thus far without success. (a) One explanation would be that it is painted red as a means of protection against insects or worms. (b) Another explanation is that the practice of painting books red began during the Glang-dar-ma’s persecution of Buddhism in Tibet. Painting books red was intended to give an impression that such books are stained with blood (and already desecrated) and that thus they would be spared from desecration and destruction. This is a popular explanation that I have heard during my monkish days in South India. In either case, painting books red may be seen as a means of protection, that is, either against malignant tiny beings or against malignant human beings. In either case, I have no written sources and my statements remain khungs med lung med. I would be thankful for any written source.
Feedbacks including from a student of mine (i.e. Tashi Wangdi), however, suggest that Tibetan books are also painted in other colors, and that color of books (like the color of hats) rather suggests a certain school affiliation. The bKa’-brgyud-pas, perhaps like the rNying-pa-pas, would rather color their books red, the dGe-lugs-pas yellow, the Bon-pos blue, and so on. Determining a relative chronology of these practices would be helpful but it would be difficult. The questions that still remain are when did these customs begin and for what reasons. Theoretically there would have been other reasons and later on a specific color would have become a symbol of school affiliation. Or, each school might have had a different reason.