Most of the galaxies in the Universe are moving away from us and as a result, the light that they emit is shifted to the red end of the spectrum due to an increase in wavelength as the Universe expands. When a galaxy moves towards us, for example in the case of the Andromeda Galaxy (M31), the wavelength of the light emitted is shortened thanks to the Doppler Effect (the same effect that causes the siren of an ambulance to rise and fall in pitch as it drives by), shifting the light to the blue end of the spectrum. The galaxy is then said to be blue-shifted.
