Final answer:
The FA call buttons typically illuminate red when smoke is detected in a lavatory, as red is commonly used to signal emergencies in aircraft.
Step-by-step explanation:
When smoke is detected in a lavatory, the FA (Flight Attendant) call buttons usually illuminate with a red indication. This is part of the aircraft's safety system designed to alert cabin crew immediately to potential fire hazards within the lavatory. Different aircraft may have variations in the color coding and alerting systems, but red is widely used across various models of commercial airplanes to signify a warning or emergency, such as smoke detection.
The FA call buttons in a lavatory illuminate red when smoke is detected. This serves as a visual indicator to alert the flight attendants that there may be a fire or smoke in the lavatory.