The Doppler effect refers to the modification in wavelength or frequency of a wave in association with an observer who is moving in comparison to the source of the wave.
The color red exhibits the longest wavelengths of any color. According to the Doppler's effect, that which has longer wavelengths moves away from the Earth, while which exhibits shorter wavelengths move towards the Earth.
Hence, the given blanks can be filled with Doppler's effect and moving away.