red sky at night...

It relates to moving high and low surface-pressure weather systems, and the way that the colors in sunlight are scattered differently by dirty and clean atmospheres. This is the explanation of how these phenomena combine to color our sunrises and sunsets.
If you look at a global map of surface pressure, you will see a string of alternating high-pressure and low-pressure areas. That is because pressure patterns are relative; i.e., if a region of lower pressure exists, it must have higher pressure on either side.
Low pressure is associated with bad weather (sailor take warning), and high pressure with good weather (sailor's delight). Low pressure causes air to converge (to try to "fill" the low), and converging air causes upward motion, which in turn produces clouds and precipitation.
In contrast, air diverges from the center of a high-pressure area. This causes downward motion, which suppresses cloud formation.
A temperature inversion (temperature increasing with height) forms at the level of the troposphere where this downward motion is strongest. This happens because downward-moving air experiences higher pressure as it descends and thus is compressed. According to the laws of physics, air heats when it is compressed. Vertical motion is inhibited at the level of the inversion; thus dirty air containing suspensions of soot, dust, and other particles (known as aerosols) is trapped near the surface.

Weather systems typically move from west to east, and red clouds result when the sun shines on their undersides at either sunrise or sunset.At these two times of day, the sun's light is passing at a very low angle through a great thickness of atmosphere, the result of which is the scattering out of most of the shorter wavelengths — the greens, blues, and violets — of the visible spectrum, and so sunlight is heavy at the red end of the spectrum. If the morning skies are red, it is because clear skies to the east permit the sun to light the undersides of moisture-bearing clouds coming in from the west. Conversely, in order to see red clouds in the evening, sunlight must have a clear path from the west in order to illuminate moisture-bearing clouds moving off to the east. There are many variations on this piece of lore, but they all carry the same message.