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.