I’m using the arc tool to create the sand mounds, but the color isn’t filling the shape correctly

Make sure the arc shape is closed. After selecting the tool, click and drag to create the arc shape; before letting go of the mouse, press “c” on your keyboard to close the shape. Alternatively, first double-click the tool (or select the tool and press the “enter/return” key) and select “close” under “type”. Then draw the closed arc shape.