The spectrum of visible light is proportional to the intervals in music (3, 3, 2, 4, 4, 5, 3) and the notes of the musical octave (Do, Re, Mi, Fa, So, La, Ti, and Do) are at the transition points between the colors (Red, Orange, Yellow, Green, Blue, Indigo, and Violet).
If we put them together, we get (Do) Red-3 (Re) Orange-3 (Mi) Yellow-2 (Fa) Green-4 (So) Blue-4 (La) Indigo-5 (Ti) Violet-3 (Do).
It is amazing that these two separate phenomena, music, and light, can be found in the same structure.
One is at the notes of the musical octave and the other is in the spaces between those notes.