Photo worth a thousand words is below. Summary: Darktable generates a lot of color noise which is really hard to remove in color photography. Why is that? How to fight it?
In Darktable 3.X (I'm using 3.8), I get very satisfying results for removing color noise completely and the remaining grain reasonably by using a single instance of Denoise (profiled) set to mode: wavelets and color mode: Y0U0V0.
The module in this mode is split to two sections:
- Y0 is for removing grain altogether;
- U0V0 is for removing color noise (making it monochrome noise which is much less annoying).
I set the Y0 curve to the same amount in all levels coarseness while the U0V0 curve is set progresivelly to remove none of the most coarse color noise and most of the fine color noise. This is to prevent removing color from small colorful objects (like toys, for instance).
The faint gray line in the graph is the Y0 curve
I combine the Denoise module with hot pixels removal module.
I learned how to use the wavelets mode of the module in this video: https://youtu.be/7ZhbeXpx2W8
I just saw the question looking for something else, hence the late date.
According to the Darktable manual (and my experience), the "profiled denoise" works best when used twice: once with "wavelets" and blend mode "colour", once with "non-local mean" and blend mode "lightness". For both, I usually tune down the intensity (NOT blend opacity), until noise gets just visible (preserves details better).
Also, you might want to adjust contrast/exposition, if you really want to compare with the RT image.