Why is there a mix result for each column in the Digital Color Mixer? I mean, why can’t we mix all the colors we choose in a single result (in a single ‘result box’, should we say?) but have different results for each column? As a result, you can only mix two colors but (for example) when you try to mix red, yellow and blue to get black, you can’t. It’d be great if each addition would have added to the “single” outcome color.
(Maybe this feature could be added to the Digital Color Mixer or a separate tool that can do the job would also be nice)
The reason for every “why is” is that Cyrille Berger designed it like this in 2009, and since, the plugin hasn’t had a lot of development. The last new feature was a reset button, added by Bourumir Wyngs in 2022 and before that, a clear history button and gradient mixer by Demetry Romanowski. Before that, there has been no development on this color selector since 2009…
I don’t think it’s used by many people, but since maintaining it (that is, adapting to other changes in Krita) is fairly easy, we haven’t removed it yet.
If someone really wants to develop it, they would be really welcome, of course!
I wish I was a developer but unfortunately I’m not 
I believe a similar tool working like the color picker’s mixing percentage but on a rectangular mixer tool with a “single outcome” would be great. Just choose the intensity of the colors you want on some columns, add them to the existing color by clicking on them and that’s it. This would also teach a lot about digital and general color theory.
One last thing. Please don’t remove it. If it can be made better, that’d be great. But it’s still so useful as it is. 
However, UNDO and REDO buttons and a reset, that’d also reset the sliders to the beginning would be great.
Ah, we won’t, since it’s easy enough to maintain. New development needs volunteers, though!