Do the green/white/red lamps on the outside of the subway entrances really mean anything?
The globes near the subway entrances were originally intended to have significance. They still do; generally speaking:
Green: Entrance open 24 hours.
Red: Entrance closed at night.
As Randy Kennedy explains in his book Subwayland, when this system was implemented in the early 80s, the intention was to also have yellow globes, signifying entrances that only had part-time token booths, but the advent of the MetroCard meant that most entrances could have MetroCard turnstiles installed.
Then, in the mid-90s, it was thought that the red and green globes didn't give off enough light, so the half-moon style globes emerged, with red or green tops and white bottoms. (Contrary to commonly held belief, the half-moons don't signify anything different from the globes that are completely red or green.)
And, naturally, as subway stations are rennovated and booth attendants are taken away, but the globes stay the same, the light system is not necessarily indicative of the status of the entrance anymore, so the above rules are not set in stone. Just another one of the subway system's eccentricities.
"The joke going around when these things were first installed," said Larry Furlong, an amateur subway historian, "was that green meant go in, red meant don't."
"And yellow meant take a cab."