The background is stored in the registry. There are a few different ways to set it. But there several ways to set a registry value via the command line.
To set the background to a specific color you need to unset any wallpaers, and set the color.
HKEY_CURRENT_USER\Control Panel\Colors\Background = ‘r g b’
HKEY_CURRENT_USER\Control Panel\Desktop\Wallpaper = ‘