When you select your drawing tool (Freehand for example) there are two color boxes where you can set the line color (that leftmost of the two color boxes) and the background color (by default it's shown as transparent which is represented by a checkerboard pattern). If you click on the background color and select a color, it will then fill in what you draw with the background color chosen. It isn't as simple as a paint bucket/fill-color tool, but it does work.
