Which traffic signal color means vehicles must stop?

The color that signifies vehicles must stop is red. This color is universally recognized as an indication for drivers to come to a complete halt at traffic signals. The purpose of the red signal is to ensure safety at intersections and other critical areas by regulating traffic flow and preventing collisions. When a driver sees a red light, it serves as a clear instruction to stop and wait until the traffic light changes, ensuring that pedestrians can cross safely and that other vehicles can proceed only when they have the right of way.

In contrast, green indicates that it is safe for vehicles to proceed, yellow serves as a warning that the light is about to change to red, and blue is not traditionally used in standard traffic signals to convey stopping or go signals.