Otacon and Drebin gave us briefly of how green collar workers involvement in the war economy of MGS world...
Weapon dealers, weapon collectors, soldiers, mercenaries, etc...
"Green-collars", because military drab uniforms used to be green, still debatable...
Take Drebin as an example, he supplied weapons to the militia to fight PMC... also he collects and unlock weapons...

Note: I dont think its the same meaning as what we read the term in the newspaper/websites articles...

Solid Snake: "War has changed."