If you have colour calibrated in Pixinsight and you used the image as the white reference, or the Nebula, this will cause the slight colour imbalance. If you have an image of a galaxy in it's Linear state you can use a preview around the galaxy as the white reference. Even tough it has no relation to the nebula your working on, you just need a good white reference.
If all your galaxy images have been stretched just make a new RGB image from a data set, and keep the linear galaxy for use in all your processing.
Uncheck the detect structure box as your using a preview of the galaxy so this is not needed.
From experience on M27 getting a good background sample is not easy as the surrounding area is just full of stars. Really expand the image so you can get in-between these. I'll use several preview boxes. If you go to scripts in the top toolbar you'll see utilities. Open that and select the preview aggregator tool, this will add all your previews together for your background. I do this for the background neutralisation. Close the aggregated image but keep the previews. Re-open the Aggregator tool and reselect the previews. You'll see they have a slightly different hue to them. Then you can open your galaxy and make a separate preview on that as your white reference.
Long winded but it should make your stars the correct colour.
Nice M27 Micheal hope this helps a bit.
Pat.