Eyedropper Tool

The Eyedropper Tool defines the foreground / background color.

In the Toolbox, select the Eyedropper Tool.
Click somewhere on your image to define the foreground color. Look how foreground selector on the Toolbox changes.
Alt-click (click holding down the [Alt] key) to set the background color. Look how the background selector on the Toolbox changes.
On the Options bar you can change sample size (the Eyedropper's range). The Point Sample picks up the color from the pixel you click on; 3 by 3 and 5 by 5 read average values from larger areas (movie).