When nonsynesthetic subjects are asked to match visual lightness with either auditory pitch or loudness, they often produce matches very similar to the correspondences reported by synesthetic subjects.
Cytowic (1989) argued that nonsynesthetes actually engage in a great deal of synesthesia, but that those synesthetic processes normally do not reach consciousness.
Lighter visual stimuli were judged to fit better with higher auditory pitches and darker visual stimuli were judged to fit better with lower auditory pitches.
Hubbard, T.L. (1996). “Synesthesia-like mappings of lightness, pitch and melodic interval.” The American journal of psychology, 109, pp. 219–238.