The Doppler effect (artist's impression)

The Doppler effect explains why objects moving towards us or away from us at high speed appear to have their colours shifted either towards blue or red respectively.

When an object moves towards us, the crests of the light waves we see from it are compressed together, making the wavelength of the light shorter (and hence bluer), while for an object moving away the separation between crests is stretched, making the light's wavelength longer (and hence redder). In this simulation, the monochromatic source of light, as it moves right, would appear blue to an observer on the right-hand side, and red to an observer on the left.