The Grey is a fictional concept featured in comic books published by DC Comics, some of which are printed under DC's Vertigo line. It relates to the Swamp Thing family of comic titles. The Green is the name attributed to a fungal aspect of The Green, which is an elemental force that governs all plant life on the planet.
At the beginning of creation, the master of the forest lords, Sila, seeded the planets with potential for life. In the Sol system, the seeds took root on three different planets: Earth, Jupiter, and another which has since been destroyed. On that last doomed planet evolved a collective fungal consciousness who thrived on the frequency of The Grey.
On Earth, plants evolved through The Green. The Parliament of Trees was formed to protect The Green, selecting an elemental to act as guardian for its generation. At the time of the thirteenth such guardian, a meteor from the destroyed Grey planet landed on earth bringing fungal life with it. The elemental Matango volunteered to be overtaken by The Grey in order to learn more from it on behalf of the Parliament. Unfortunately, The Grey's drive to spread, consume, and break down its hosts aligned well with Matango's desire for power, and they formed a collective consciousness on Earth in opposition to The Green.
Though The Green and The Grey coexisted in relative peace for a time, the dawn of man and its potential for destruction led Matango to tempt them with fruit from Tuuru, the Tree of Knowledge. The result prompted a war between the two life forces; a war which weakened both sides and forced the Parliament to leave the Garden of Eden to the ice and snow, and move to Brazil.
While Matango lay dormant for eons after, fungi continued to coexist with flora as the result of a compromise. However, when the Swamp Thing escaped from the Claw of Aelkhünd, and fathered a human plant elemental, Matango grew agitated, and sent emissaries of The Grey to capture the Swamp Thing's human daughter Tefé.
Realizing the danger, Swamp Thing began reviving those trapped since the Great Migration by The Grey, in opposition to Matango.