ROYGBIV

ROYGBIV or Roy G. Biv is an acronym for the sequence of hues commonly described as making up a rainbow: Red, Orange, Yellow, Green, Blue, Indigo and Violet.

A rainbow spans a continuous spectrum of colours; the distinct bands are an artifact of human color vision. In ROYGBIV, the colours are arranged in the order of decreasing wavelengths, with red being 650 nm and violet being about 400 nm. The acronym is memorable because Roy is a common male given name, so Roy G. Biv sounds like a first name, middle initial, and a last name.

from Wikipedia