§ 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 greenCleared for takeoffCleared to land.
Flashing greenCleared to taxiReturn for landing (to be followed by steady green at proper time).
Steady redStopGive way to other aircraft and continue circling.
Flashing redTaxi clear of runway in useAirport unsafe—do not land.
Flashing whiteReturn to starting point on airportNot applicable.
Alternating red and greenExercise extreme cautionExercise extreme caution.