MIKE MOSS SAYS: Larry, You're quite right that an otherwise beuatiful sunset can be dressed up even more by the right kind of cloud cover. This is often, as you noted, in the form of streaks of high cirrus clouds, but there can also be very vibrant colors and patterns associated with patches of mid-level clouds like altocumuouls and altostratus.
The general idea for why sunsets have a red or orange cast is the flip side of reason a clear, clean sky is blue. Air molecules preferentially scatter the shorter end (blue/indigo/violet) of the visible light spectrum more than the longer wavelengths (red/orange), in a process called Rayleigh scattering, so that when you look in some other direction than the sun, you are seeing mainly blue light scattered toward you, while the converse occurs near sunset. In this case, the light traveling in your direction from the western sky and from the area near the setting sun is passing through a deep layer of air (skimming in your direction through the lower atmosphere) and the blue light is being scattered off to the sides so that what remains for you to see is shifted toward the red end. This same reddened light is being reflected off some of the mid or high level clouds, but as they are at varying altitudes, angles and distances from you and from the sun (which can be above or even below the horizon), the particular colors can be shifted, or light from one direction can be combined with light being reflected or scattered from a different direction, allowing for combinations that can paint a broader pallette of resulting colors.
An added element of variety arises from the presence or absence of pollutants (natural or man made) in the form of gases or particulates that can modify the scattering and color variations, with effects that can differ depending on the concentration of the pollutants and the size of the suspended particles relative to the wavelengths of light. These particles can further be mixed uniformly over a deep layer of the atmosphere or can be confined to one or more fairly shallow layers, and can be spread over a large horizontal area or channeled in plumes, adding to the possible variety of visual impacts on a sunset or sunrise scene.
That's a very brief answer about a very complex subject, but hopefully gives you some sense of how all those different shades can arise.
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