Messiaen is the strongest documented case of chromesthesia in the classical canon. He described his experience precisely and often: chords produced complexes of color, always the same for the same harmony, transposing through shades as the chord transposed. He annotated scores with color instructions, described his modes of limited transposition in terms of their hues, and gave detailed color descriptions of specific sonorities in interviews and his own theoretical writing.
He was explicit that the experience was inward perception rather than stage effect, and impatient with performances that ignored the colors.
Works like Couleurs de la Cite Celeste and Des canyons aux etoiles carry the trait in their titles and construction. For researchers and musicians alike, Messiaen is the reference case: a major composer whose entire harmonic system doubled as a report of music-color synesthesia.