My 1988 ACSC comments in the associated notes that there are various shades for most stamps in this set. Here are some examples.
My only other copy of ACSC comes from the 1990's by which time the decimal issues were in a swparate volume. Do any of the later versions of ACSC comment further on the color shifts. For example, are the original issue's colours specified, colour changes dated or reasons given for the sometimes radical colour shifts.
As a bit of a postscript, some of New Zealand's 1970 definitive set, eg the 25c and 30c values, printed by Bradbury Wilkinson, had a similar range of colours changes.