Tone-Color Correspondence to a Double Octave

In this position, indigo creates a harmonic center for the perception of color. This color symmetry likely evolved according to the same harmonic principles found in sound where the ninth partial in the harmonic series acts as a harmonic center for the formation of harmonic overtones. For example, in the key of C major the ninth D acts as a harmonic center of symmetry for the diatonic scale. This Tone-Color Correspondence is thus the only symmetrical assignment of the three color cones of the human retina to the harmonic series and diatonic scale of common practice music tradition.