If you open the drawing toolbar, you should see a paint tin icon, hover over that and you will see that it is labelled fill color. If you click on it you can select colours, effects and even use a picture to fill the shape! (click on the fill effects and then the picture tab and browse to the picture you want to use).
You can also use the other drawing tools (such as shadow, 3d, line style, etc. to good effect). Myself and my two boys have spent many rainy afternoons just messing about with just the drawing tools, speech bubbles, text boxes and a few pictures!