In another thread I wrote this in January about the color zones module and making B&W/monochrome photos:
I can easily target many colors to get different shades of gray with color zones (or the Lightroom HSL/Color module where I can adjust 8 colors to get different shades of gray: red, orange, yellow, green, aqua, blue, purple, magenta). I guess we all know that there are actually just red, green, blue BUT having more editing choices is easier to hone in on colors that are not themselves pure red, green, or blue. Most colors are combinations of R, G, B so, personally, I find it easier to have more choices.
The color zones module with the color graph is quite nice. It would be even better if, like the tone equalizer, one could hover the mouse cursor over a color and then turn the mouse wheel to brighten/darken it though.
Something you can do though which is about as good is you can take the eyedropper and select something in the image and then a black vertical line will appear on the color graph. You can then move a node up or down along that vertical black line to adjust the shade of gray.
This thread: