Idle question: Color of tape coatings

While working on some old (again!) half-inch tapes, I note that some of the very old ones have an oxide coating about the color of milk chocolate. Newer ones are anywhere from dark chocolate to black. Reel construction is another aspect. The really old ones tend to be all clear plastic, including the hub area. Newer ones have either a black plastic reinforcement to the hub or employ an aluminum sleeve. In most cases, the oldest of these is from around 1964, but probably older than that, as the only clues I have are dates placed by the tape librarian when a tape is put back into the pool or a label indicating when the tape was last recertified. Was there a date after which *all* half-inch tape became the dark brown to nearly black in color? --Chuck