I began shooting in 1967 with federal paper 20s, and they were yellow then. But you could still see some 20s, I seem to recall Remington/Peters, that were not, so I suspect the 1966 date for at least the beginning of the change was spot on. I recall an experienced shooter explaining the change was because 20s were being dropped into 12s, and then another shell could be loaded behind it and fired. He explained that the 16s would not drop through a 12 chamber, and so they didn't have the same problem. FWIW.