Ideally, something neutral in color and a flat finish. There's nothing wrong with a room with white walls but you're going to get a lot of reflection and bounce that may not always be desirable. You could also have a dark room or one with black walls but that's just plain gloomy and uninviting, and since it's not going to bounce much light at all so in that respect any and all light has to be created by you. With a gray wall you're somewhere in the middle, allowing some light to bounce around, but in a very controlled way.
Don't sweat over the color being middle gray or 18% gray. That's not important, and your walls are not going to be used for white balance or exposure, although in a pinch I have used my gray wall as my reference for custom white balance and it worked well.