Spectrum Of Rainbow Light

A rainbow spans a continuous spectrum of colours; the distinct bands are an artifact of human colour vision. The most commonly cited and remembered sequence, in English, is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G. Biv). Rainbows can be caused by other forms of water than rain, including mist, spray, and dew.
Visibility
Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind at a low altitude angle. The most spectacular rainbow displays happen when half the sky is still dark with raining clouds and the observer is at a spot with clear sky in the direction of the sun. The result is a luminous rainbow that contrasts with the darkened background.