If you select that check box and make no other select related to that feature, the result that you get depends on the theme specified for the layout. The fill color specified for the alternate and active row states are initially specified by the currently selected theme.
But you can select the portal while in layout mode and go to the appearance tab in the inspector. Pull down the uppermost drop down and change the selection from "portal" to "portal row". Then use the drop down immediately below the first to select either the primary, alternate or active row properties for the portal row so that you can then use the appearance controls to specify different fill colors and borders.
Bless your heart, Phil, this worked like a charm. And it took you only 3 lines of type to explain it thoroly. Why is it that the FileMaker documentation couldn't have done as good a job? (That was a rhetorical question; I don't expect you to have to explain it.)