What color must clearance lights on the rear of a vehicle be?

Clearance lights on the rear of a vehicle must be red. This requirement is based on safety regulations that dictate the colors to be used for various types of vehicle lighting. Red lights are universally recognized as signals indicating a vehicle's presence from behind and are critical for ensuring visibility and safety during nighttime or low-light conditions.

Using red for clearance lights at the rear helps to communicate specific information to other drivers, such as the vehicle’s width and position on the road. This is particularly important for larger vehicles, which may have attachments or extended parts that could pose hazards if not clearly marked.

In comparison, the other colors mentioned may serve different functions or may not comply with legal standards for clearance or marker lights at the rear. For instance, green lights are typically not used in standard vehicle lighting, yellow may indicate caution, and white is usually reserved for headlights or reverse lights, thereby making red the correct choice for this specific application.