When using a Wild Growth Rune.
On Open PvP and Retro Open PvP worlds, its color varies depending wether the caster and the viewer are on an active PvP situation or not. On Hardcore PvP and Retro Hardcore PvP worlds it's always dark, whereas on Optional PvP worlds it will always appear with the lighter color.
It is similar to the Magic Wall (Object), although one can shoot through a wild growth wall and not through a magic wall. They can be destroyed with Machetes, Heavy Machetes, and any of the Secret Agent Tools on all PvP worlds, and by walking over them on Optional PvP worlds.