For practical purpose, synthetic luminance generation assumes that, besides possibly varying total exposure lengths, all other factors remain equal. E.g. it is assumed that bandwidth response is exactly equal to that of the other filters in terms of width and transmission, and that only shot noise from the object varies (either due to differences in signal in the different filter band from the imaged object, or due to differing exposure times).
When added to a real (non synthetic) luminance filter source, the synthetic luminance's three red, green and blue channels are assumed to contribute exactly one third to the added synthetic luminance. E.g. it is assumed that the aggregate filter response of the individual three red, green and blue channels, exactly match that of the single luminance channel.
This is a very basic workflow using defaults, showing how the new Compose module (replacing the LRGB module in StarTools 1.5) makes complex LLRGB compositing and processing incredibly easy.
The two aspects - color and luminance - of your image are neatly separated thanks to StarTools' signal evolution Tracking engine.
The desired crop is created by clicking and dragging with the mouse the area to retain.
As of the 1.4. beta versions, StarTools stores the masks you used in the StarTools.log file.
StarTools is famous for its Color Constancy color rendering.
You can convert everything you see to a format you find convenient. Give it a try!