[Al Skierkiewicz]

Green is the standard color in the US for ground. Electricians understand that the green screws connect to earth somewhere but do not actually represent a good earthing connection for equipment that actually requires earth for some particular reason. (RF suppression for instance) Electrical codes differ widely in different parts of the country and in different settings. Hospitals for instance carry very strict electrical codes for patient safety when connected to electrical monitors and other equipment. Your house may have green wires running to outlets, electrically bonded metal conduit and metal boxes, or a bare wire in romex or BX Cable (sometimes called Greenfield). Most codes call for that wiring to connect to a metal water pipe or ground stake near the electrical box.
As pointed out, there is no 'ground' on the robot. When coloring the hardware, be aware that the hardware may be providing electrical connection.