There is an article in January’s “Sky at Night” magazine under “Skills” and “Image Processing” entitled “Improve your deep-sky images with luminosity layers”. While not entirely BS, it seems to me that it is a complicated way of doing something relatively simple. There is nothing new about the point that most of the resolution information in a colour image is in the luminance part of the image. After all, this is how colour TV signals have been transmitted for ever. With colour TV, the image is decomposed into a luminance, or black and white signal and a chrominance signal that contains the colour information. Luminance is then transmitted on a high-bandwidth channel, and the chrominance on a low-bandwidth sub-carrier. They are combined again inside your TV receiver.
The author of the article used Photoshop, but you can do essentially what he did (and to me, much more simply) with GIMP. How to do it is in an article from 3 years ago on our own web-site at http://www.thornett.net/Rosliston/Astrophotography/DSO.pdf.
AND, of course, as we keep reminding everyone, GIMP is FREE!