The Sports of the Great Explorers stamps are unusual, but not unique by any means. They were excluded from the stamp sheets available to collectors, being printed in sports-only sheets. This resulted in sports in a much brighter and easy to spot shade. Why was this? There was no mention of a sport-less sheet in the announcement, nor any later illumination from the Emporium. It may have been deliberate policy, but why should that be, and why the shade difference? A reason may have been to limit the number of sports, as there were only 16 stamps per sheet. Even so, the colour variation is still unexplained.
Could the omission have been accidental and only noticed later when sports only sheets needed to be printed in haste, and the shade could not be matched to the originals?

Anyway, here are the stamps and their sports in their gaudier shades.