eli5: At sundown, why does the sky have a gradient of several colors? Is it 2 colors blending and naturally creating the other ones as a result from mixing?
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There is a different amount of atmosphere that the light travels through. When the sun is above you, that is the shortest amount of atmosphere that light passes through to get to you, at sunset and sunrise the light is passing through a lot more atmosphere, gradually changing it’s appearance
the color of light coming from the sun isn’t composed out of red, green and blue; it’s a continuous spectrum from red to violet, so the colors in the sunset sky are the same white light but with slightly different properties attached, parts of the light passes though clouds, parts have to reflect around the earth’s curvature, parts get red shifted by the atmosphere, etc
if you’re interested in this more you should check out how a prism refracts light and imagine both the atmosphere and the clouds as huge prisms
The atmosphere acts like a prism, or a lens, and the light is diffracted (“bent”) to different amounts depending upon wavelength. We don’t see the colors when the path is more or less perpendicular to the layering of the atmosphere, because the bending is near zero for all light, never mind wavelength. It is only when incident angles are high that the bending by wavelength separates the paths enough for us to perceive the different colors (the dominant wavelength of the light we are seeing makes the color, it is not really a mixing process).
There is also a much longer path for light through the atmosphere than if it is coming straight through, so the bending that occurs way near the top of the atmosphere has a long way it can go to force the different wavelengths to spread out (make colors visibly different by location). Blue end (short wavelength) bends more than red end (long wavelength) so when the sun is near the horizon, the blue has already made it down to the ground further toward the sun (late afternoon over there tot he west), and all that is left a far distance to the east from the source is red-end light. As you move from the horizon upward through the sky, you will pass from red through the mid-colors to blue, because the more-bent light is coming from nearer to you in the sky. The red part of that light is passing further toward the east for someone else to see as a red sunset.
We also do not see the colors when scattering is extreme (this leaves blue for kind of the reverse reason that red is all that is left from direct bending), which is why we perceive the sky as having a blue color during the day.
The last light from the source at sundown (and first at sunrise) is basically only red. Moisture and temperature layering in the atmosphere also affect the refraction (how much the light bends and scatters), so some conditions are FAR superior to others for making this color segregation occur.
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