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.
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.