Olivier Messiaen's published descriptions of his works and the color labels that appear in certain of his scores show that he is affected by colored-hearing synaesthesia. Because Messiaen's color responses, like those of other synaesthetes, exhibit a high degree of internal consistency, the analyst may tabulate the available correlations between sound and color and use them to explore the various factors, objectively considered, that govern Messiaen's color associations. The importance of absolute pitch and of the modes of limited transposition is studied, as are the conditions under which vertical spacing and pitch-class-set identity may assume primary significance in color delineation. The conclusions reached offer a key to more general matters of structure in Messiaen's music. Examples are drawn from several of Messiaen's works, dating from 1929 to 1974.