OK, read it.
Short version is that he airbrushed the plastic "glass" with 1 or 2 light passes of a 1:1 mixture of Alclad II Lacquers ALC404 "Transparent Green" and ALC403 "Transparent Blue". That gave him the tint that he felt matched the prototype for vintage rail cars.
Those lacquers are apparently still available at multiple sources like Hobby Lobby, RCPlanet, eBay, Walmart, etc.