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).