New to Photoshop CS is a Histogram palette that continually updates itself as you work on your image. A Histogram provides a graphical representation of the tonal and color information in your image.
Click on Window . The Window menu will appear.
Click on Histogram . The Histogram palette will become active.
Click on the Histogram palette menu button . A list of options will appear.
Click on Expanded View . This will expand the size of your histogram and provide you with additional color and tonal information about your image.
Make a color or tonal change to your image. In this example, we are adjusting the color balance. As you make changes, they are reflected in the histogram. The previous histogram appears in light gray, and the new histogram based on the changes you are making appears in black.
Click on OK in whichever tonal or color change dialog box you are using. As soon as you finish applying your color or tonal changes, the light gray histogram disappears, and you are left with the black one.
Click on Navigator in the palettes window to return to the Navigator palette.