There sure is ...
If you select the portal object and go to the inspector -> appearance tab ... under the style you will see portal in a dropdown type box, if you select that you can change it to Portal row and then in the dropdown below that you can change to active, this will allow you to set a fill for the active portal row.
Is this limited to FM13? When I select the portal object there is no dd type box in FM12 that I can see.
It may very well be, I can't recall if it was available in 12 ... sorry!.