Why does stacking reduce star color?

Question

I attempted to photograph the Big Dipper last summer. While I am very happy with the overall result, I recently found that the color of the stars has been greatly reduced during stacking. The following photos show about the same section of the sky.
Single Light Frame:

Unprocessed Stack output:

Final Image:

As you can see, the violet color of the two stars is greatly reduced and almost lost in the final image. For the difference between the latter two, this can be explained by an (apparently) too strong brighntess increase of the stars (I should have limited this to the fainter stars).
I stacked 75 Light Frames (ISO 1250, f/2.8, 4 seconds), 25 Dark Frames, 30 Flat Frames and 30 Bias Frames. In terms of post-processing, I mainly adjusted levels, curves, removed a gradient, increased star brightness and selectively increased saturation for some stars.
What could be the cause of the loss of color when stacking?

Some more information in respond to the comments: The two bright stars are Mizar and Alkor. Micar (the lower, larger star) is Spectral Type A1V, Alcor is A5V+M3-4V.
The violet-ish color is present on every light frame.

xenoid · Answer

In some other answer in these parts it was pointed out that stars can also be small enough to fall onto a single sensel so they can be red, green, or blue (or missing one of the channels) depending on where they fall on the sensor. Stacking images would average their position and mitigate this, removing the color bias.

Answered by xenoid on February 10, 2021

Sherwood Botsford · Answer

Suppose that Mizar had a colour value 20:5:20 so the red and the blue were stronger than the green giving you a violet star.
Stack up 10 copies:  200:50:200.  Same ratios.
Stack up 20 copies  400:100:400.  But your rgb values are limited to a max of 255  So you end up with 255:100:255
Stack up 50 copies and you end up with 255:250:255.
Now the stacking software is not this simple minded, but this illustrates the problem.

user85781 · Answer

The image is more accurate, not less accurate.  The star color is not str color at all.  It is an aberration.

Why does stacking reduce star color?

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