Editable objects have (optional) background color to fill their inside shape.
 The <Attribute>:<Background>:<Visible On/off> menu button toggles background
 painting.

 Background colors can be changed individually, or on selected sets of
 objects altogether, through any color button in the <Attribute>:<Background>
 menu. Colors can be reverted to original (<Attribute>:<Background>:<Default>).
 Color buttons in the menu are assigned through X11 resources

atg*background<int>  (for instance atg*background5 : green ), and

 their number must be provided as: 

atg*nbBackgrounds : <int>

 The default background value for each object type is set by resources, as
 well as a background for highlighting them. Resources names are:

atg*boxBackground
atg*hilightBoxbackground

 and similarly for vertex, port, edge, web instead of box.