Final answer:
A green tag is placed on the main control valve of a fire protection system to indicate it is operational and without defects.
Step-by-step explanation:
When a fire protection system is restored back to operational status, the color tag that would be placed at the main control valve to indicate that the system is fully operational and free of any defects is green. The use of a green tag is standardized within the fire protection industry to communicate that the system is operational and has been inspected and approved for use. The green tag signals to firefighters, maintenance personnel, and safety inspectors that the fire protection system is in good working order.