It isn’t pure white and is not shiny, but it is reflective (the whole point of a movie screen). It is mesh-like fabric which allows light to pass through from behind as well. The material is called ‘oxford nylon’. Google that and you can see close-up images of the screen. This image shows a few colors. I think ‘B4’ is the closest color:
it is more of a dull gray with a slight sheen.
For more information please check out our guide setting up a great backyard setup
Or take a look at our recommended projectors and screens