What color background with white font indicates a mandatory instruction sign at a runway holdline?

A mandatory instruction sign at a runway holdline is indicated by a red background with white font. This specific color scheme is crucial for pilots and ground personnel to recognize essential information that mandates compliance, such as stopping before entering an active runway.

Red is universally associated with stop signals and warnings, thereby drawing immediate attention to the instructions conveyed on the sign. This helps to prevent incursions onto runways and enhances safety within the airfield environment. The bright white text contrasts sharply against the red background, ensuring maximum visibility, even in less-than-ideal lighting conditions.

Other colors serve different purposes: blue is typically reserved for information signs (like services or locations), green often denotes guidance to taxiways, and yellow is used mainly for caution or advisory messages, indicating areas where caution should be exercised. Understanding these color codes is vital for safe navigation and operation in airfield environments.