In playback mode, you can display the histogram for the picture by pressing down on the thumb switch.
This will show a single histogram. To make the histogram go away, press up on the thumb switch.
You can also view a histogram for each color separately. In order to do this, press OK on the thumb switch to call up the Retouch Menu, choose Filter Effects and then choose Color Balance.
Pressing the play button takes you back to normal playback mode.
