Dan Schroeder's response indicates to me that I should have added
a comment about sunsets: not all sunsets are red. If there are few
scattering particles along the line of sight the Sun will appear
too bright to look at; it will be colorless. Scattering by the air
molecules is insufficient to dim the Sun, so scratch that as an
explanation. When Mt. Pinatubo exploded we had red sunsets every
clear evening for quite a while, but when one gets away from the
sources of particulates the Sun is no longer red at sunset.