Relations between music and color have long been debated, it seems inconclusively. I describe the theory behind three electronic sound-to-light transducers built to give a visual impression of music to students and deaf people. In psychophysical theory, any psychological correlation between music and color must derive primarily from the physical stimuli, which, as sonic or radiant energy, have only two variables: (a) amplitude, causing loudness or brightness/lightness: and (b) wavelength, causing musical tone or hue. Correlation of tone and hue is also indicated by their cyclic nature, as octave cycle and hue cycle. Accordingly, only the latter cycle can represent the former.