Lindsay - If you add a colored rectangle to your page, then only the colored rectangle will appear on the screen surrounded by whatever your page background color is (white by default).

But if you Ctrl drag and drop a color onto the screen, then the entire browser screen will be that color. You can also select a tiling image from the Bitmap Gallery and press Background and that pattern will fill the browser.

See this thread for an example. http://www.talkgraphics.com/showthread.php?t=36974