green light (1937/1952)

“Eddie [Brandt) was given a green light by Pie Traynor, coaching at first.” “Break of the Game,” Pittsburgh Sun-Telegraph, September 29, 1937, p26
2nd definition: A sign given to a baserunner to take an extra base or attempt to steal

“‘But he was a bit outside so I went to a 3-0 count. Charley (Dressen) had me take a strike, but gave me the green light on the next pitch and I was lucky enough to single.” Tommy Holmes quoted in Jack Barry, “Holmes Sees Bright Future for Several Braves Rookies,” Boston Globe, July 6, 1952, p53
1st definition: A coach’s sign flashed to a batter allowing him to swing. It is most commonly given on a 3-0 count.

No earliest use given in 2009 Dickson Baseball Dictionary