The Contrast brush
            increases or decreases the difference between light and dark pixels
            on a texture map in the areas that you stroke.
         
         It works in a manner
            similar to contrast tools in other 2D image editing applications.
         
         NoteThe Contrast tool
               only works on pixels on the active paint layer. That is, it does
               not have any effect on the underlying shading material color.
            
         
            - Click the Layers tab
               to display the Layers window, and set the
               layer display to Paint.
            
- In the Layers window,
               select the paint layer whose contrast you want to edit.
            
- In the Paint Tools tray,
               select the Contrast tool.
            
- In the Contrast tool
               properties window, set the Contrast property
               to either increase or decrease the difference between light and
               dark pixels.
               A positive Contrast value
                  increases the contrast and negative values reduce it. The range
                  is -10 to 10. The default setting is 1.
                
- Stroke across the painted texture in
               the areas you want to affect.
               Contrast is modified
                  in the areas that you stroke.