1. 'at', 'on' or no preposition. We don't use 'yellow' for the middle light in BrE; it's 'amber' or, for some people, 'orange'.
2. The traffic light was never at/on (or no preposition) green everyspacetime (when) I was there.
3. Stop (the car) on/at red
light/at/on the red light
'Traffic light' is fine; we also commonly use 'traffic lights'.
- For Teachers