Final answer:
The usual color for emergency line couplers depends on the country and organization.
Step-by-step explanation:
The usual color for emergency line couplers depends on the country and organization. In the United States, emergency line couplers are typically red or yellow. Red is commonly used for fire-related emergencies, while yellow is used for general emergencies. In some countries, emergency line couplers may also be blue or another color specific to their emergency services system.