dust in the atmosphere, persistent evening clouds, the amount of moisture in the air -- all are contributors to the formation of varying colors (I love a salmon colored sunset...) and the swirling clouds picking them up and altering them... yeah, I like sunsets -- don't ask me about sunrises until mid December! (northern hemisphere)

f

Red sky at morning, sailors take warning.
Red sky at night, sailor's delight.

Humidity, dust.

Atmospheric conditions and orientation of sun in relation to earth’s axis and rotation.