It's a little bit tricky for beginners. You have to drag the circle on the right of the dropper (Pipette) symbol to the place of the image (or even the screen) where you want to pick the colour. Then release the mouse key. The circle near the dropper symbol will turn to the new colour. Then select the gradient point you want to redye and click on the circle. The gradient point will get the new colour then. The dropper symbol itself is not a button. It is only a hint that the circle is the colour picker.