Every color of light is a musical note — just 40 octaves too high to hear. Play a note: hear the sound, and see the color it becomes when you double its pitch forty times. Red is a G.
honest physics: take a note's frequency and double it 40 times (×240). It leaps from something you can hear into something you can see — visible light. The seven colors of the rainbow are these seven notes, raised forty octaves. Measured honestly the rainbow isn't a sweet major chord — it's rooted on G and carries the blue notes inside it.