Navigation:  Painting Tools > Color Picker

Color Picker tool can be activated through the main menu -> Paint -> Color Picker.

Or you can click at the following button on the Painting Tools panel:

(you can open this panel through the main menu -> Paint -> Show Paint Tools).

After that, you need to click somewhere on the image to select a color and use it for some other drawing tools e.g.  brush, pencil, fill.

Warning! The image layer must be selected in the layers list in order to use color picker.