The ECAM message NW STRG DISC changes from green to amber after:

The ECAM message NW STRG DISC changes from green to amber after the first engine start because this indicates a transition in the status of the nose wheel steering system. Initially, if the nose wheel steering is functioning properly, the message displays in green, indicating that the system is operational. However, following the first engine start, there is typically a change in hydraulic pressure and systems status, which can affect the nose wheel steering functionality. Since the nose wheel steering is primarily powered by hydraulic systems, the amber indication serves as a warning that highlights the potential for a performance shift or a change in the system's reliability after engine start.

This status change alerts the crew to verify the condition of the nose wheel steering and take appropriate actions if necessary. A nose wheel steering failure or disconnection on its own would lead to specific messages, and a green hydraulic failure would affect multiple systems not just limited to nose wheel steering. Thus, the proper understanding of operational status changes is critical for safe aircraft operations.