The Eyedropper tool 
is useful to pick colors from the image itself. It can be activated
moving the mouse on the Eraser icon in the toolbar:
Alternatively you can activate it pressing the I key or Alt+click.
Using Left click will pick the color and store it as the
Foreground color, using
the Right click it will be stored as
the Background color.
You can configure the Right click to pick colors from
Edit > Preferences > Editor.
SEE ALSO