Was that just coincidence because the 15th and 16th did go though the full 5 colours on their own,
but the 17th didn't..?? e.g.
15th: 1981 Red, 1983 Green, 1984 Yellow, 1986 Blue, 1987 Brown..
16th: 1991 Red, 1994 Green, 1997 Yellow, 2001 Blue, 2004 Brown..
17th: 2008 Red, 2011 Green, 2015...