The Howondaland 50 Wichit Bead stamp was issued in the Year of the Complicated Monkey and was printed by Teemer and Spools in Ankh-Morpork for use by intrepid explorers of the Ankh-Morpork Trespassers Society to send mail from the port of Zambingo.

The quite intense printing of deep brown or dark umber with a woodcut design may have presented printing problems, for at least four shades are easily recognizable, at least when sat next to each other.

      
Brown                                          Slightly red                                      Reddish                                           Deep

The differences seem to be degrees of mis-registration of the red. Close-ups of the top left of the stamps, shown here in the same order as above, sort of confirms this.
Sheet examination shows that colour is consistent on all stamps leading me to assume that either this stamp had several print runs, or printing was inconsistent over an extensive print run. It follows that each shade variation has its own sport.