Color replacement tool.
Blacking out/shading is easily done with layers and reveal all masking. Create a duplicate layer, then add a third layer that is sandwiched between those two, then fill it with black. By playing with the opacity of the 2nd layer, you get a simple blacking/greyed out of the area that you revealed using the reveal all mask on the top layer. It works best by making the bottom layer a black and white image.
Once you get it figured out, you can get creative by using black/white gradients on the reveal all mask and get something, for example, like
this image that I did of my son, or like
this one I did of one of his team mates.
