AIM - RAC 4.3.11 Visual Signals
 

Visual signals used by the tower and their meanings are as follows:

TO AIRCRAFT ON THE GROUND:

1 SERIES OF GREEN FLASHES Cleared to taxi.
2 STEADY GREEN LIGHT Cleared for takeoff.
3 SERIES OF RED FLASHES Taxi clear of landing area in use.
4 STEADY RED LIGHT Stop.
5 FLASHING WHITE LIGHT Return to starting point on airport.
6 BLINKING RUNWAY LIGHTS Advises vehicles and pedestrians to vacate runways immediately.