Converting to B&W can easily be done like shown here:
Using HSL’s pickup/dropper tool, you can select the colour and then widen or narrow the selected range by dragging the inner and outer handles in the HSL ring. Note that this procedure is a selective rather than a local adjustment possibility.
Also try the local adjustment “Control Line” tool. It might also do what you want, not by using the colour mixer, but the dropper tool and the sliders that already exist.