I have had some issues with getting the correct colours to a number of images, some my own, some not. With my m31, which I posted in the gallery, I'm assuming there was so much noise (including walking, streaking noise) that this interfered with StarTools. So the colour calibration was totally manual. In Jim Misti's m101, the data is obviously very good, yet I'm still having problems. Here is a link https://www.flickr.com/gp/rkonrad/9Q83N5. The outer areas of the galaxy is slightly purple. StarTools here a mixture of red and blue, mostly blue. Is this correct?
An edit: This example shows the purple (almost red) more clearly. Jim Misti's data of m51. https://www.flickr.com/gp/rkonrad/v3bb21 I'm aware that greedy trying to show the nebulocity - maybe the purple is the result of noise?