easiest way is to make the color on a seperate layer, copy the color layer onto a 3rd layer (so two layers have the color) use the chisel tool and color it a dark or light color that u want the shading /lighting to be and then turn down the opacity and make the shading layer semi- transparent, thenturn the opacity back up to reach the transparency that you want the non shaded areas to be.

i could show you some examples but the best example i have is porn :SKULL:

if you look close at the mask area you can see the color i used for the shading is a reddish color. but dont use solid black and white, it can mess with the shading itself depending on the color of the character and accents. it genuinely depends on what it is that you are shading