GiantColorSwatch shows a window containing a swatch of a color. This can be used for backdrops, testing Color Panel extensions, and possibly other things that I haven't thought of.
You can change the color using the Color Panel, which is shown for you on launch. You can close or miniaturize the Color Panel to get it out of the way; if you close it, you can get it back again with “Show Colors” in the Edit menu.
You can also move or resize the window. If you close it, the application will quit.
When the application quits, the color will be saved. When it next launches, the color from the previous session will be restored.