Red sky at sunset or sunrise is due to Rayleigh scattering: when the sun is low, its light passes through a much thicker atmospheric layer, and short wavelengths (blue, violet) scatter almost completely, leaving only reds and oranges to reach the observer.
Sailor's rule
"Red sky at night, sailor's delight; red sky in morning, sailor's warning" has a meteorological basis: a red sunset indicates dry air to the west (good weather approaching), while a red sunrise may mean the dry air has passed and moisture approaches from the west. Red intensity increases with suspended particles (haze, volcanic ash).