It's the stuff that picks you up when you're at the bottom and makes you keep on going even when you're sure you can't. That's the green.
"Browns seek knowledge, Blues meddle in causes, and Whites consider the questions of truth with implacable logic. We all do some of it all, of course. But to be Green means to stand ready. In the Trolloc Wars, we were often called the Battle Ajah. All Aes Sedai helped where and when they could, but the Green Ajah alone was always with the armies, in almost every battle. We were the counter to the dreadlords. The Battle Ajah. And now we stand ready, for the Trollocs to come south again, for Tarmon Gai'don. the Last Battle. We will be there. That is what it means to be Green. -Alanna"
― Robert Jordan