green light

(noun): A traffic light in its green state (indicating that vehicles may proceed).
(noun): Approval, or permission to proceed.
(noun): A go-ahead to commit a crime, specifically to carry out a hit on someone (i.e. to kill).

The drive home was much faster than usual; somehow we managed to hit every green light.
We have a green light on the project. Work starts today.
Yo man, I heard someone had put a green light out on you, so watch your back.