What indicates that the oxygen system is functioning properly?

A green line flow indicator signifies that the oxygen system is functioning properly. In aviation, color coding is utilized for clarity and safety, making it easier for crew members to quickly identify the status of various systems. The green indicator specifically denotes that oxygen is being supplied as intended and is readily available for use, ensuring the safety and readiness of the crew and passengers in case of a cabin depressurization or similar emergency.

The other indicators, such as red, blue, and yellow, typically represent different statuses or warnings about the system's condition, suggesting caution or an issue needing attention. Therefore, the presence of a green line provides a clear and reassuring signal that the oxygen system is operational.