This corresponds to the BGCOLOR="..." HTML tag attribute on the <BODY> tag.
You can modify this value at any time, the result of which will be to change the background color of the page.
The background can be colored whether an image is loaded into the background of a document or not. In fact, it may be advisable to set the background color to something similar to the average color of the background image in case the image takes a long time to load or the browser is unable to display a background image.
The background color for the document that is defined in the <BODY> tag is also reflected in the bgColor property of the document object, although that is now deprecated as of DOM level 1.