Final answer:
The usual color of the tractor-to-trailer service brake line is red, which is standardized for quick identification during maintenance or emergencies.
Step-by-step explanation:
The usual color of the tractor-to-trailer service brake line is red. This color coding is important for safety reasons and to ensure that drivers and mechanics can quickly identify the correct lines during maintenance or in the case of an emergency. Other lines, such as the supply line, may be coded differently, often in blue or green.