Kandinsky’s writings are saturated with cross-sensory description: colors with sounds, timbres with hues, cello as deep blue, and a formative account of seeing the music at a Wagner performance. Concerning the Spiritual in Art reads at times like a synesthete’s field notes, and his stage composition The Yellow Sound scripted music and color as equal voices.
The dispute is whether this was perception or program. His correspondences shift between texts in ways stable synesthesia should not, and he was explicitly building a theory of art that needed the senses joined. Scholars split, with careful reviews landing on unproven.
We file him as disputed: possibly a synesthete, definitely the most influential describer of the experience in art history. The full context sits in our Kandinsky and music entry.