1. In the Toolbox, select the Color Replacement Tool.
2. On the Options bar, set brush size and style.
3. Paint to replace a color.
4. In the Options bar, you can choose a Sampling mode*:
   • Continuous (default method) replaces all the colors with the foreground color.
   • Once only allows to replace color, that you first click on; for example, if you start a stroke on a red area the tool replaces only red pixels.
   • Background Swatch only replaces pixels containing the current background color.
5. Also on the Options bar, you can define blending mode, Tolerance (the tool's sensitivity) and Limits - the tool's range: all over the image (Discontiguous), only the adjacent areas (Contiguous) or the pixels inside only (Find Edges).

*In Photoshop 9.0, you select sampling mode with the "continuous" , "once" and "background swatch" buttons in the Options bar