14 CFR § 91.125 - ATC light signals.

§ 91.125 ATC light signals.

ATC light signals have the meaning shown in the following table:

Color and type of signal Meaning with respect to aircraft on the surface Meaning with respect to aircraft in flight
Steady green Cleared for takeoff Cleared to land.
Flashing green Cleared to taxi Return for landing (to be followed by steady green at proper time).
Steady red Stop Give way to other aircraft and continue circling.
Flashing red Taxi clear of runway in use Airport unsafe—do not land.
Flashing white Return to starting point on airport Not applicable.
Alternating red and green Exercise extreme caution Exercise extreme caution.