An operator of the city traffic light maintenance service receives a call from a citizen to communicate that one of the traffic lights that regulates the passage of cars on the main avenue has a fused light. The operator who answers the call, asks his interlocutor what the light is cast but unfortunately the person who reports the problem is colorblind and is not able to determine the color of the light that does not work. However, he tells the operator that it is the light at the top of the traffic light.
What color is the light bulb that the operator must carry to repair the traffic light?
The operator must carry a conventional white light bulb since the color of each of the lights of the traffic lights is determined by the glass that covers the bulb.