Definitely a color changeling. I have seen this on that and similar stamps. If you look carefully, you can still see the mauve tint on the left stamp.
Hi Ian, I'd guess it's a colour changeling though I'm not certain. There seems to be a mauve wash on King Luis's head on the blue stamp which isn't present on the mauve one.
re: Mozambique Company, Unlisted Colour? Reprint or Forgery?