Greens prize their ability to stay calm in an emergency.
When others are off being emotional, they remain in absolute control of their faculties.
It's illogical when, at the moment of crisis, some people disengage their brains and let their feelings run amuck.
Yes, little Johnny may have cut off his thumb, but that's no reason to wail and bop around like a decapitated turkey.
So the unflappable Green quietly takes charge, sends someone to phone for help, has Johnny lie down with his hand up in the air, then applies a tourniquet created from Johnny's shoelace and a toothbrush.
Meanwhile, the Green has directed another bystander to locate the emancipated digit and place it in a plastic bag on ice.
As Johnny's weepy father is rambling on about how Johnny will never be able to hitchhike again or save a gladiator's life, and how he would gladly sacrifice his own big toe if the surgeon can't reattach Johnny's thumb, the Green is speaking calmly to Johnny and adjusting the tourniquet at regular intervals.

Always behave like a duck -- keep calm and unruffled on the surface but paddle like the devil underneath.
- Jacob Braude

Of course this calm demeanor doesn't only surface during emergencies.
This is a Greens' normal operational status.
They prefer to remain as cool, calm, and collected as possible.
Even if they just discovered how to turn lead into gold, they might be as giddy as a schoolgirl inside but as cool as a glacier on the outside.
They have learned to keep their emotions under wraps so feelings don't interfere with their objective analysis.