In the Finder, create a new folder somewhere on your hard drive. Size it to fill the screen, set the view to Icons, set the View Options to use a background color of your choice, and hide the toolbar. Close the folder. Now, whenever you need the background hidden, just open your maximized folder.
[robg adds: I use either Backdrop or Desktop Cutain to hide the desktop -- they take anywhere from 10MB to 20MB of real RAM. I used to use the folder method, but found that I wanted the added features of the application solution (it's simpler to toggle between colors and images; set the level at which the hiding occurs; etc.). To me, that's worth the 15MB or so of real RAM it requires.]

