Well, it's definitely green—at first.
On the outside, the maze itself looks like any other kind of hedge maze. Nothing special about it. As kids, Alexander and his brothers were told never to go inside, though. One time, his older brother, Dominick, wound up getting lost in it, and while they didn't talk much about it, Alexander wrote it off for being trauma since they had both just lost their mother.
Really, the hedge maze has a crazy effect on the mental state of a person. They'll walk around and feel as though someone is with them. If they are with someone, they won't appear the way they were before, not to their perception. It's all about the perspective changes on the person themselves. They'll start to see and hear things. They may hear voices, or see a lost loved one wandering around but never quite catch up to them. If they have any mental traumas, it would reflect on their experience in the maze.
For instance, while Alexander's brother was lost in the maze, he would claim he saw apparitions of their mother, shortly after her supposed "suicide". In truth it's never really known whether or not if there is actually something that controls the reality shifts within the maze itself, or if it's entirely self-induced by the person wandering within it after a long period of time.
For another example, if James Sunderland were to get lost in it, he would probably run into some Silent Hillish experiences. Some of the monsters might appear, or he might just hear them and not see them anywhere at all. Conversely, he might also see pleasant or unpleasant images of his dead wife within it, maybe run into completely altered sections that remind him of Silent Hill.
Again, the maze is based purely on the perception of the one who ventures into it.
Eventually, the sky starts to change a different color, and you start to see things swimming and climbing around it. Everything becomes monochromatic and there becomes absolutely no color at all. It's like walking right into a black and white film.
You never really find your way out of the maze. Something actually takes you out, but you never quite know who or what it is.