Newer versions of Ubuntu Desktop have removed the capability to set the background to a solid color. You may run the following command in a terminal window to accomplish this task:
To clear the current image setting:
> gsettings set org.gnome.desktop.background picture-uri ""
To set the primary color (the first color in a gradient or the solid color):
> gsettings set org.gnome.desktop.background primary-color '#000000'
To set the secondary color (the second color in a gradient, ignored for solid):
> gsettings set org.gnome.desktop.background secondary-color '#000000'
To set the background type ('solid' for one color, 'horizontal' or 'vertical' for gradient):
> gsettings set org.gnome.desktop.background color-shading-type 'solid'