What color must a motorcycle's stop lamp be?

The stop lamp on a motorcycle must be red or amber. This requirement is consistent with safety regulations that aim to ensure visibility and clarity in signaling to other road users. A red stop lamp is traditionally used to indicate that a vehicle is slowing down or coming to a stop, making it easily recognizable and conveying a clear message to drivers and pedestrians. Amber, while less common for stop lamps, is also an acceptable color used in some signal applications to indicate caution or alert other road users of the vehicle's actions.

Using colors like green, blue, white, or clear for a stop lamp would not provide the necessary visibility and could lead to confusion on the road. Each of these colors is typically designated for other functions on vehicles, enhancing the importance of adhering to standard color coding for safety devices. Therefore, red or amber is the correct choice for a motorcycle's stop lamp to ensure maximum visibility and effective communication with others in traffic.