What color gives a truer perception of a vehicle's motion compared to red?

The correct choice is blue because it provides a higher contrast against common backgrounds, improving visibility in various conditions. This color perception can enhance a driver's reaction time to the vehicle's motion. Additionally, blue light has been shown to be more effective in certain settings for alertness and clarity, thus better communicating the presence and movement of emergency vehicles to other drivers.

While green, yellow, and amber are also visible colors used in emergency situations, they do not convey the same level of clear perception of motion as blue does. Green may blend more with natural surroundings, yellow is commonly used for caution but might not be as attention-grabbing, and amber, while effective for signaling, doesn’t present the same contrast. In emergency services, the ability to perceive a vehicle's motion quickly can significantly enhance safety and response times.