@gwidion The easiest way to understand it, is to go to https://www.photopea.com and load a colorful picture in it. Then, use Layer - New Adjustment Layer - Selective Color. Then play with it. Photopea has cloned it from Photoshop, so even if you don't have photoshop, you can investigate it there. It's the best way than trying to understand it second-hand via articles or videos.
@eugenialoli Hi - I am late building a working GIMP 3 build environment on my computer - But I checked the functionality online -
Yes, it is a matter of building a GEGL plugin targeting the appropriate channels. Once one is more or less proficient in it, it should take a couple of hours (I am not, yet) ,I don't think layer effects in GIMP 3 can be individually masked, though - the workaround for that is having a layer copy, with a mask, and apply the effect on that.