Update visualisation of primaries sliders to look similar to that in AgX #19727
Description
Prompted by #19642 (review)
AgX uses an inaccurate, exaggerated, but highly visible visualisation for it 'primaries' sliders (hue and purity). Would it make sense to use the same for rgb primaries and sigmoid?
What would most probably make sense, even if the visualisation is not updated, is setting feedback = 0, so the white overlay does not obscure the slider. This method is already used for some sliders.
color balance rgb:
AgX:
