In Opticks (1704), Newton divided the spectrum into seven bands, red through violet, and matched their proportions to the intervals of a musical scale, arranging both around a circle. Why seven colors? Largely because the scale had seven notes; orange and indigo owe their canonical status to the analogy.
The gesture founded three centuries of color-music speculation. Castel built on it within decades, and key-color charts from Rimington to Scriabin descend from its central move: mapping the octave onto the spectrum.
Physics eventually declined the metaphor (light doubles in frequency across one octave of the spectrum, nothing like sound’s ten), but as art-historical DNA the circle is everywhere the genre goes.