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.

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I describe complementary colors' physiology and functional roles in color vision, in a three-stage theory (receptor, opponent color, and complementary color stages). 40 specific roles include the complementary structuring of: S and L cones, opponent single cells, cardinal directions, hue cycle structure, hue constancy, trichromatic color mixture, additive/subtractive primaries, two unique hues, color mixture space, uniform hue difference, lightness-, saturation-, and wavelength/huediscrimination, spectral sensitivity, chromatic adaptation, metamerism, chromatic induction, Helson-Judd effect, colored shadows, color rendering, warm-cool colors, brilliance, color harmony, Aristotle's flight of colors, whiteblack responsivity, Helmholtz-Kohlrausch effect, rainbows/halos/glories, dichromatism, spectral-sharpening, and trimodality of functions (RGB peaks, CMY troughs whose complementarism adapts functions to illuminant). The 40 specific roles fall into 3 general roles: color mixture, color constancy, and color perception. Complementarism evidently structures much of the visual process. Its physiology is evident in complementarism of cones, and opponent single cells in retina, LGN, and cortex. Genetics show our first cones were S and L, which are complementary in daylight D65, giving a standard white to aid chromatic adaptation. M cone later split from L to oppose the nonspectral (red and purple) hues mixed from SþL. Response curves and wavelength peaks of cones L, S, and (SþL), M, closely resemble, and lead to, those of opponent-color chromatic responses y, b, and r, g, a bimodal system whose summation gives spectral-sharpened trimodal complementarism (RGB peaks, CMY troughs). Spectral sharpening demands a post-receptoral, post-opponent-colors location, hence a third stage.

In recent research, it has been increasingly necessary to employ an extended wavelength metric to cover the complete hue cycle so as to research or represent data as a function of relative wavelength rather than psychological scales such as CIELAB or Munsell hue angle. This article describes such a relative wavelength metric and its derivation from complementary wavelength functions. The metric provides a useful psychophysical, wavelength-based, ratio scale for the hue cycle allowing nonspectrals to be treated in the same coherent scale as spectral hues. Several indicators, e.g., color order hue cycles, infer the (optimally efficient) spectral hues comprise about 71% and the nonspectrals about 29%, of the hue cycle interval. This gives a hue cycle whose relative wavelength interval is about 240 nm for illuminant D65. To relate to CIE colorimetry, spectral complementary wavelengths to the nonspectrals' relative wavelengths are identified for seven illuminants including D65, D50, C, and A. Data are given for CIE 1931, and briefly 1964, colorimetry.

four wavelengths are 'invariant' for a given L level or ratio. With higher L level, invariants shift as opposed pairs: to longer wavelength for b and y, shorter for g and r. With higher L ratio, b and y invariants shift some 10 nm longer but g and r remain about 508 nm and 493.5 c. With higher illuminant color temperature, invariants (1:10 cd/m2) shift shorter for b and y, r slightly longer (spectral to nonspectral 494 c) but g seems constant. Hue-shift data for real object color are very similar to aperture color. The full hue cycle is graphed to the recent extended wavelength scale for CIE dominant wavelength. Many effects are newly reported.