The existence of an idea of a missing shade of blue contradicts Hume's first principle that simple ideas all derive from corresponding simple impressions. Hume dismisses the exception to his principle as unimportant. Why does he do so? His later account of distinctions of reason suggests a systematic way of dealing with simple ideas not derived from simple impressions. Why does he not return to the problem of the missing shade, having offered that account? Several suggestions as to Hume's solution (...) of the problem of the missing shade (not all appealing to distinctions of reason) are explored with an eye both to their adequacy as Humean solutions and their value as clues to his dismissal of the problem. Hypotheses concerning the latter perplexity are formulated and discussed as well. Senses in which the missing shade of blue is or may be a red herring are identified. In course, this author names Hume's missing shade marjorie grene. Historians of philosophy will want to adopt this nomenclature. (shrink)