A mnemonic used to help you remember the order of colors in a rainbow, which is also the order of the colors when sorted by wavelength (which would be the inverse of frequency)

From long wavelength:

  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet

Also apparently conforms to the color code of resistors when ordered by their resistance, with the addition of black before red, and grey and white after violet.