From Adorama’s 100 Photography Tips in 100 Days:

Adobe Photoshop includes a wonderful tool called the History Brush. It allows you to Undo virtually any single operation that has been performed on an image—and do it selectively, via carefully applied brushstrokes. When combined with certain other tools and/or filters, the History Brush enables you to create unique images that will amaze your friends.

For example, if you open an image file and follow this sequence: Image > Adjustments > Desaturate, all of the color will disappear from the image. Next, select the History Brush from the Toolbar (or type “y” on the keyboard) and use the tool to “paint back” the areas of the image where you want to restore the original color. The result is an image that appears to have been shot in black and white and then partially hand-colored.

To see an example, click here.