Sponge tool

You can use the sponge tool to paint in localized adjustments that will modify the saturation of an image. The sponge tool has two modes: Saturate increases the color saturation, while Desaturate decreases the color saturation.

As with the dodge and burn tools, if your goal is to increase or decrease the saturation in a large area of a picture, the best way to go about doing this is to add a Hue/Saturation adjustment layer to boost or decrease the saturation of the entire image, fill the layer using black to hide the layer contents and then paint with white on the adjustment layer mask using a soft edged brush. Filling with black will hide the adjustment layer effect and painting with white will allow you to selectively reveal the adjustment layer effect and as with all other layers, you have a layer opacity slider that will allow you to fade the overall layer opacity.

Note that in Photoshop CS4 or later, the sponge tool also has a Vibrance option in the tool options bar (see Figure 1). This means that you can increase or decrease the saturation using the same advanced logic as the Vibrance adjustment.

Figure 1 The sponge tool Options bar.