Green means go means just that - you get off the bus, walk into the rental car facility, go to the left side escalators, pick your car, and drive out. That is if you're an Emerald member for National.
The cars are pretty good - clean, spacious, and never disappointing. The process is pretty quick - just drive up to the kiosk, finalize a few details, and you're off to Lost Wages.
However, as a note, I'd build in an extra 30 minutes for turn in since you have to catch the bus back to the main terminal on your way home. Otherwise, you risk missing your flight with the long walk in, long security lines, and tram to get to the gates.