SJAvery

Personally, I would not extract a Luminance master from the debayered image and therefore would not be using LRGB to recombine the Luminance again!  I don't quite see the point of doing that and it is not conventional processing.  As a beginner, I would try to keep things simple and concentrate on the essentials:  get good data, get good calibration frames, find a good way of calibrating (personally I like to use PixInsight's own batch processing script).  Then remove the light pollution, adjust the background, adjust the colour calibration and then try reducing the noise with TGV.  Don't worry about using masks until you know what you're doing.

Hope that helps!

Without knowing the exact processing sequence you are using, it is difficult to be precise.  However, your image does look very noisy.  Also, I see that you turned up the colour saturation quite a lot when using LRGBCombination, which may account for the colour cast.  At what stage did you use HDRMT?  That needs to be used very carefully, and usually near the of your process.

How much data did you have? 

Stephen