A green flag or light is specifically used as the "right away" signal to the driver. Where a guard is on-board, only they can give this signal. There is an important safety aspect. A green light or flag between the station staff and guard could be seen by the driver and interpreted as "right away", when the guard is not ready.
This is where teenage me - Misinterpreting the CD indicators at various SR stations of the time - Starts to regret making jokes about GWR stations still using „8-track“ indications... 🚈📼💿🙃 Edit: ...But I understand the „LP 33“ indicators used on the Island Line are starting to regain popularity again. 😋
