346R: Medieval prince feigning illness

Looking for a children’s picture book from the 1970s. It was about a prince trying to get out of something. He draws red spots on his face to pretend to be ill. It’s set in medieval times, as he wore tights and those pointy slippers/shoes that turn up at the ends. That illustration or the cover was purple. Hardcover, portrait and about 8x10” or letter size. Might be British.