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
Left click will pick the color and store it as the
Foreground color, using
Right click it will be stored as
the Background color.
You can configure the
Right click to pick colors from
Edit > Preferences > Editor.