Kill one of the men that has green on. You will see a man in yellow walk out in the open and run back. After a moment, the man in the yellow will run over to you. When he does this, and grab him. If another man runs with him, kill him and leave the man in yellow alive. Take him to give the voice authorization and you will be able to open gate. When you go over to the gate, a lot of men will be there. An easy way to kill all of them is to run up to them and grab one as a shield. The man you grab will get killed quickly. Keep doing this until only one person remains.