It would be nice if tabs, frames, rectangles, etc. supported background colors. It would be even better if the sktch-mode rendering of the colors supported the same hand-drawn style as other stencils ie. a chalk-like scribble of color.
Jack Josephy commented
Wireframes arn't meant for visual design. They represent the functionality and layout of a page. You will merely mis-represent the look and feel of the finished product to your client if you start to show them aesthetic design ideas in a wireframe. Use photoshop or GIMP to build a visual prototype.