x1 is to the red of x2; of a wave: x1 (wave, color, etc.) has greater wavelength/lesser frequency than x2 (wave, color, etc.) in medium/under conditions/in frame of reference x3.
Infrared is to the red of red and all human-visible colors; red is to the red of orange; orange is to the red of violet; violet is to the red of ultraviolet. Not confined to just colors or electromagnetic radiation: low-pitched musical notes are to the red of high-pitched musical notes. See: "zi'irpu". Could be useful for referencing the concept of redshift, or appropriate Doppler shift.