The Saturation Painting tool allows you to selective paint in or paint out saturation in any area of your image without having to find the correct sliders in the Hue & Saturation adjustment layer.
This tool works by creating a transparent layer, on top of the rest of your layers, and changes the blending mode to Saturation. By changing the blending mode to Saturation we’re telling Photoshop, anything we paint on this layer will only affect the saturation in your images and nothing else.
Your foreground and background colours will also change to a purely saturated red and a middle grey. If you want to add saturation to an area of your image you will use the red colour. It’s not going to paint red on your image, it’s using the saturation level of the colour to determine whether it adds of removes saturation, so it could be any saturated colour. When using this tool, drop the opacity of your brush to on or below 5%, the effect is quite strong, and paint over an area multiple times if you want to increase saturation further.
Use the grey brush if you want to decrease saturation in area, using all of the same steps shown above when using the red brush.
If you have painted in to much, or gone into areas that you didn’t want affected, rather then trying to erase the painting, put a mask on the layer and paint with black brush to hide the effect from the areas you want to remove.
The video below will show how to use the Saturation Painting tool in more detail.