You can fill an area freehand-style with the Filled Stroke brush. We don't have a tool to automatically fill an area with a color, it's something we'd like to enable in the future.
There are a couple workarounds. If you have drawn the object with a single stroke, you can select the stroke and change its tool to Filled Stroke to convert it. (iOS users - In addition to freehand drawing, this works great for shapes drawn with the Shape Guides, and various objects in the Object Market (not all are enabled for stroke conversion but basic shapes, figures and other simple objects respond).)
You can create quick outlined strokes by first drawing the Filled Stroke, then selecting it, duplicating it, and converting the duplicate to an ink stroke (such as Fixed Width, a helpful pen for drawing consistent line work for presentations, mind maps, etc). You can drag the fill or ink to a separate layer if you'd like more control over the individual elements of the final shape. This works vice versa - Ink to Fill, as well.