WordNet (r) 3.0 (2006):

green light
    n 1: a signal to proceed [syn: green light, go-ahead]
    2: permission to proceed with a project or to take action; "the
       gave the green light for construction to begin"

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "green light":
   John Hancock, OK, Roman candle, acceptance, admission, affirmance,
   affirmation, aid to navigation, alarm, allowance, amber light,
   approbation, approval, authentication, authorization, balefire,
   beacon, beacon fire, bell, bell buoy, blinker, blue peter, buoy,
   caution light, certification, charter, confirmation, consent,
   countersignature, dispensation, endorsement, flare, fog bell,
   fog signal, fog whistle, foghorn, glance, go light, go-ahead,
   gong buoy, heliograph, high sign, imprimatur,
   international alphabet flag, international numeral pennant, kick,
   leave, leer, liberty, license, marker beacon, nod, notarization,
   nudge, okay, parachute flare, patent, permission,
   permission to enter, pilot flag, poke, police whistle,
   quarantine flag, radio beacon, ratification, red flag, red light,
   release, rocket, rubber stamp, sailing aid, sanction, seal,
   semaphore, semaphore flag, semaphore telegraph, sigil, sign,
   signal, signal beacon, signal bell, signal fire, signal flag,
   signal gong, signal gun, signal lamp, signal light, signal mast,
   signal post, signal rocket, signal shot, signal siren,
   signal tower, signature, signet, spar buoy, special permission,
   stamp, stamp of approval, stop light, stop-and-go light,
   subscription, the nod, the wink, ticket, ticket of admission,
   touch, traffic light, traffic signal, validation, visa, vise,
   vouchsafement, waiver, warrant, watch fire, white flag, wigwag,
   wigwag flag, wink, yellow flag