Currently green is 24-hour station agent OR entrance/exit via HEET turnstile (MetroCard required). Some green entrances have MetroCard vending machines (no agents) while others are unmanned and have no machines. Red indicates exit only or entrance open limited periods (e.g weekdays only).
Back then, green was full time station agent, yellow part-time and red closed or exit only. In the 1990s green was made for 24-hour entrances while red was used for part-time booths as well.
Since my friend continues to chain smoke nonstop, she is probably an Alco.