I used Kodak 18% grey for my upper walls and ceiling - it doesn't make the place look like a dungeon, and it's excellent for not washing out the screen if any light reflects back off the walls due to it's neutral light.
For accoustics, a rule of thumb is to have the lower halves of the wall (below ear height) dead - so I used carpet. I didn't put any on the back wall,as the seating does the same job.
The top halves should be reflective, so I wallpapered those, and the ceiling.
The screen wall is black carpet, thoughyour case it's difficult to carpet a window, so do as retro suggests.
Pics on my website if you're interested.
Gary.