There is too much math for me here, even though I feel I should have understood Chris' post.
Anyway. Here's my take on the colors of the APOD:
So I take it that the brown color of the exhaust is the natural color of whatever is in there. I assume that the reddish, yellow and white parts of the top of the plume are reddish sunset light, yellow sunset light and full white sunlight. I take it that the pink part of the sky is the Belt of Venus, and the dark blue part of the sky below the pink part is the Earth's shadow.
What is most striking to me, of course, is the intensely blue color of the shadow of the shuttle plume. Could the color of the shadow have anything to do with the blue hour?
Wikipedia wrote:
The blue hour (from French l'heure bleue; pronounced [lœʁ blø]) is the period of twilight (in the morning or evening, around the nautical stage) when the Sun is at a significant depth below the horizon. During this time, the remaining sunlight takes on a mostly blue shade. This shade differs from the colour of the sky on a clear day, which is caused by Rayleigh scattering.
The blue hour occurs when the Sun is far enough below the horizon so that the sunlight's blue wavelengths dominate due to the Chappuis absorption caused by ozone.
I hope you're taking notes.
Anyway. Shadows are often very blue on sunny days when there is snow on the ground, as in this photo from Malmö on January 18, 2024:
The shadows are very blue here, as you can see.
But are shadows in the sky really blue for the same reason as shadows on snow on a sunny day (when shadows get their color from the blue light of the sky)?
Help, Chris! Why is the shadow of the shuttle plume so blue in the APOD? Is it, perhaps, just a consequence of the processing?
Ann