I am using Darktable 3.0 on Windows, but I guess it is the same situation on other platform.
Check my sample image, the top one is without color zone applied, the middle one is with color zone applied (to quite extreme extent to demostrate the issue). As you can see the “color noise” is very obvious after I tweak the lightness and saturation, I am not sure it is real color noise or not, because if I turn on “display selection”, the mask and the shape of noises are quite aligned. Plus the picture itself only has ISO 180.
In parametric mask, I can increase feathering radius to smoth the selection, but here I I have no way to do the similar way.
Any suggestion how can I make sky not so noisy?