Under the 4 ways tab in color balance rgb it is not possible to remove chroma for a specific hue, chroma can only be added. I don’t understand why, because I think it would be a nice option. What is the reason that this is the case? And which tool do DT users use when they want to reduce chrome in a specific color area?
Jan.
The 4-ways tab is about brightness ranges. You can colour-grade them (add a hue, or remove a cast: you select the hue to add, and the chroma to set how much you add). So, it’s not a place to boost or reduce the saturation of colours.
You can use global desaturation and apply a parametric mask; or you can try other modules, like color zones (you may have to enter the name in the search field, if your module layout preset does not include it).
You probably know that, but it would be kinda redundant, because reducing the chroma for a specific hue is the same as adding chroma for the opposite hue. So, allowing for “negative” chroma would be more of a comfort function. Also, there may be implementation specific limitations 
That’s my understanding.
thats not correct for darktables 4 ways tab: 4 ways tab tints with a given hue and chroma - a negative chroma value isn’t related to a kind of opposite hue.
if you want to deal with chroma, then use the proper tools designed for this purpose.
a car with 2 2 wheels not mounted isn’t a proper substitute for a motorcycle
I don’t think that’s what Jan wants to do. He specifically asked: ‘which tool do DT users use when they want to reduce chrome in a specific color area?’
The 4-way is not a tool to manipulate (increase or decrease the chroma) ‘in a specific color area’. It is to alter the overall appearance of colours for specific brightness ranges. See darktable 4.2 user manual - color balance rgb
To reduce / increase chroma of colours:
Probably a misunderstanding on my side, or I didn’t find the right words to express myself correctly. But let’s set this right…
So, can we settle on the following: one job of the 4ways tab is, to remove color casts? (you know motorcycles, wheels and so on…). Also @kofa writes above:
You can do this by selecting the opposite hue (or using the color picker, to select it for you) and then you add the amount to counter your color cast.
My understanding of @janvangastel was, he would like to directly select the hue of the color cast and then reduce an appropriate amount (instead of adding the opposite hue). I know, this is not possible as of now - but at least theoretically this should work (shouldn’t it?). And I stumbled across this myself.
Feel free to tell me where I’m wrong. This is my experimental understanding of how this works.
I think only @janvangastel can tell if one of the explanations got his intention right. 
Yes
Adjust the hue by 180 degrees and you are effectively removing it as you add chroma… but its as others said more of a tool for adding or removing color casts to the various tonal region’s… To address aspects of a specific hue you can use color zones and further use masking if that is not giving you the correction you need…
Yes, this is what I meant. I now do that in Affinity Photo , on the .tif file I export from DT after raw editing. If I have, for instance’ a photograph with heather (moor, heathland, don’t know the right translation of the Dutch word ‘hei’) it is often a bit too yellowish. I then change the reds a little by pulling back the yellows and add some magenta with a tool named ‘selective color’.
Well you could try the color zones module and use the picker…target a hue and adjust the hue or the saturation of that hue… its quite effective and might save you an export step …
One tool I use often is if I have an overt cast or say even an overly rosy cheek or sometime you just get an area with a bit of yellowish dinge…then I select the saturation tab in the CZ module and then holding shift and the autopicker I draw a selection on the area it will do an auto desaturation. I does a nice job… you can do the opposite and ctrl click to boost it… but I find I use the desaturation one quite a lot… It is also easy to boost or pull back the effect with the mix slider… It does a great job…
Sounds like what you want is actually the channel mixer in color calibration (r, g, b tabs). Takes a little getting used to, but is very powerful.
Some good tutorials:
Thank you, I will watch the video’s and try it out.
If you want to correct a colour cast, due to several different light sources illuminating the scene, yet another option is to use several masked instances of color calibration with the chromatic adaptation defaults setting set to ‘modern’ (or switching white balance to camera reference manually). That allows you to pick one white balance for each instance, and control which white balance is used where using masks.
darktable 4.2 user manual - processing “auto-apply chromatic adaptation defaults”
https://www.youtube.com/results?search_query=darktable+color+calibration
Aurélien Pierre is the author of color balance rgb, color calibration and also filmic. His videos often have harsh language, but they are technically good.
@s7habo Boris Hajdukovic and @Bruce_Williams Bruce Williams Photography, @Rico Rico Richardson are active on this forum as well, so you can ask them directly (and I know there are others).
I use color zones a lot to increase or decrease the saturation or lightness of a specific color.
As for your original question about why this is not possible in the color balance module, I remember the developer explaining this in one of his videos, but I can not remember if it impossible to correctly implement or just undesirable in his view.
It would also be possible to use any module that reduces chroma or vibrance and limit the effect to specific hues based upon a parametric mask. If you are not already familiar with parametric masks I would recommend taking a look at this very powerful option.
Good luck.