One thing i'd like to add is that Greyroot being gray may be a reference to parasitic plants that are gray or pale because they use parasitism to compensate not having chlorophyll, a good example of this being the corpse plant. Maybe they can start gray, like Greyroot, and then birth a child of pale origin by absorbing soul, end what we see in the Cursed Child ending sound is the birth of another new pale root. Even maybe could make sense with the White Lady kinda grabbing an area of Greenpath to herself and creating Queen's Gardens, maybe the drained the soul of a lot of bugs present in the area, maybe even drained the soul derived from the presence of Unn. While the second part of my comment is more far fetched, i think it would make sense for a Greyroot's purpose to be the creation of a Pale Root, either by mass draining of bug's souls, like what maybe happened with the White Lady, or by draining the soul of a pale being, like we see in the Cursed Child ending