The Capur Mortuum pigment, sometimes also known as the cardinal purple, is an artistic name for metal hemaxite. The name of the pigment most likely comes from the alchymists‘ researches aka it presents the rest of the metal oxide that is unable to be used furthermore. The colour was used back in the Middle Ages for dress portrayal in religious paintings and on the portraits of famous people.
As for the German aviation, this pigment was used as a part of the more-colourful top camouflage (p.e. Albatros).