What color in MUTCD signifies regulation?

In the Manual on Uniform Traffic Control Devices (MUTCD), the color that signifies regulation is black. This color is primarily used for regulatory signs, which include stop signs, yield signs, and other signs that convey mandates that must be followed by motorists and pedestrians. Regulatory signs are crucial for maintaining order on the roads and ensuring safe and efficient traffic flow by informing drivers of the rules that govern traffic behavior.

Yellow is typically used for warning signs, indicating caution without indicating a mandate. Blue signs are often used to convey informational messages, such as those related to services or points of interest, but do not carry regulatory authority. Red, while indicating prohibitory signs such as stop signs or yield signs, does not represent all regulatory signage since it is limited to specific contexts where a complete halt or significant caution is required. Thus, black is the correct choice for indicating regulation in the context of traffic control devices as described in the MUTCD.