Astronomers observe light in specific wavelengths with a diffraction grating, a glass surface with fine grooves cut into it. A measurable shift in the wavelength of a traveling wave caused by the relative motion of the source and observer. On the straight-aways of the Indianapolis 500, the sound of cars whizzing by an observer changes pitch from high to low. Light behaves in somewhat the same way. Stars with relative motion toward an astronomer appear blue, i.e. the entire spectrum of star light shifts toward the blue end of the spectrum or is blue-shifted.
Stars speeding away from the astronomer appear red, i.e. the entire spectrum is red-shifted.