Place the cursor inside the pad and click the LMB to start drawing. By clicking again you will disable the drawing function which will allow you to move the cursor inside the pad without applying the color. Click once again to enable the "brush".
By double-clicking you will randomly select a color of your "brush".
By using the pallete above the pad you can assign a specific color either to the "brush" or the pad itself (by choosing a color and clicking the "Set As Background Button").