x1 is to the violet of x2; of a wave: x1 (wave, color, etc.) has lesser wavelength/greater frequency than x2 (wave, color, etc.) in medium/under conditions/in frame of reference x3.
Red is to the violet of infrared; orange is to the violet of red; violet is to the violet of all other human-visible colors; ultraviolet is to the violet of violet. Not confined to just colors or electromagnetic radiation: higher-pitched musical notes are to the violet of lower-pitched ones. See: "xu'unre". Could be useful for the concept of blueshift, or of appropriate Doppler shift.