The sky is pink at sunrise thanks to Rayleigh scattering: the phenomenon causes photons from the Sun's rays to disperse when they encounter gas molecules in the atmosphere.
Scattering is more intense for shorter wavelengths: that's why the sky is blue during the day. Sun rays travel longer in the atmosphere during sunset and sunrise, and blue and green wavelengths get scattered well before we can observe them. This leaves the relatively unaffected red hues to reach us, giving us the warm colors of dawn and dusk.