Hi Larry,
For the LRGB combination task I recommend you to use our new LRGBCombination process that we have implemented in the new PixInsight Standard application. We have a specific tutorial here:
http://pleiades-astrophoto.com/tutorials/The PixInsight Standard application is still in the beta development stage, but it is fully functional and very stable. You can download the latest beta here:
http://pleiades-astrophoto.com/download/STD/Of course you have to calibrate, register and stack your individual RGB and luminance frames. For now these tasks cannot be done in PixInsight (though this is going to change shortly).
After stacking, you have four images: three for the RGB color channels, and one for the luminance (I'm assuming you aren't using other filters). Then you have to perform a first RGB combination to get a single color image, then a LRGB combination that yields a final combined image. Both tasks can be carried out with LRGBCombination. You should neither equalize backgrounds nor multiply these images before LRGB combination in PixInsight.
Regarding atmospheric extinction, you can introduce the corresponding factors on the LRGBCombination window. You should multiply your channel weights (perhaps derived from a GV2 calibration, or from any empirical method) by the extinction factors, then the corrected weights can be entered as Channel Weights on the LRGBCombination interface.
As for background "equalization" (we usually refer to this as
background neutralization), our HistogramTransform and Statistics tools allow for a high accuracy manual neutralization procedure that gives excellent results. This procedure should be applied to the RGB combined image (just before LRGB combination). On the above linked tutorial you can find a description; see section 2.3.
The RGB/K channels in the Histogram and Curves processes don't refer to luminance. Transformations (histogram or curves) defined for RGB/K apply equally to the three individual RGB channels of color images, or to the unique channels of grayscale images.
The Curves process has a 'L' curve which does apply a transformation to the luminance (CIE L component) of a color image. Along with L, the CurvesTransform process in the PixInsight Standard application provides separate curves for the CIE a, CIE b and CIE c components, plus a special color saturation curve that preserves the luminance (the 'S' curve).
Hope this helps.
