GTViewer and GTVx have had Background Masks (BGMask) for sometime, but they are new to the GTV .NET Control. BGMasks are a cleaver way to highlight an area in the view without covering up anything on the map. They highlight from the bottom instead of the top, so the features in the map are drawn on top of the highlighted area instead of being covered up; similar to drawing a transparent area on top of your features but with the benefit of not washing out the features with.
The BGMask are better to show than to talk about, so here are some screenshots of a demo app. Demo source code is available upon request: