Background (see https://greenlantern.fandom.com/wiki/Emotional_Spectrum#Green.2FWillpower):
Green is the color in the center of the spectrum, and represents
willpower. This color has the least amount of influence on the wielder,
and can only be controlled by one who has an indomitable will that can
overcome great fear. The
Oans harnessed this energy. Following the departure of The Controllers, those who remained on
Oa became the
Guardians of the Universe, and used the green light as the power source for the
Green Lantern Corps, an organization that would battle evil whenever and wherever it rose to threaten the universe.