From Wikipedia, the free encyclopedia. Blue (from Old High German "blao" shining) is one of the three primary additive colors; blue light has the shortest wavelength (about 470 nm) of the three primary colors. A clear sky on a sunny day is colored blue because of Rayleigh scattering of the light from the Sun. Large amounts of water (H2O) look blue because red light around 750 nm is absorbed as an overtone of the O-H stretching vibration. Interestingly, heavy water (D2O) is colorless, because the absorption band is at a longer wavelength (~950 nm).