As arenhaus has mentioned, ArtRage currently doesn't have a tool for selecting areas, nor does it have an option to simply fill an area in. ArtRage was was made to be more of a creation starting point then an image editor. Programs like Photoshop (not free, nor cheap) and Gimp (free to use, but a tad clumsy) have the options for this sort thing. You can also try Sumo free online here: http://www.sumopaint.com. Just click the sumo guy image to open the program straight from the site.
It should be noted, if the scanned line work isn't nice and crisp before being imported one can't expect to simply click and be done with the coloring (regardless of the tool being used).
Nothing is easy to the unwilling.