Ground screws do not need to be green.
Though they sell some very cute green ones ten for a buck if you want them to be green.
Ground screws must be -32 thread pitch or finer, and must be threaded into the metal box. 10-32 is the "conventional" size. And most metal boxes have a hole tapped for a #10-32 screw for precisely that purpose.
This is visible as a hole slightly smaller than the normal mounting holes.
If you can't find it for love or money, likely someone has driven the physical mounting screw into that hole. Better boxes try to help deter this by putting the ground screw on a stamped "bung". That also gives the ground screw a place to go besides straight into the wall, but if someone misused the hole for a mounting screw, that problem will be solved :)