I have Kodak 18% grey on my upper walls and ceiling, and similar carpet colour for the side walls (red floor carpet too.

). However, my attic ceiling is close to the screens top corners (see pics in my link if you're interested) and I recently measured the ANSI contrast and it's suffereing because of the walls being slightly reflective near the screen. I'll paint the walls and ceiling black for about 6ft from the screen and cover the carpet in black speaker grill cloth so that the accoustics remain the same but the light will be less reflected. I'll then measure again, but black felt would be the best solution for absorbing light spill and retaining intra-scene image contrast.
When you have light or reflective walls/ceiling, if you have a scene that has both light and dark elements in it, the light parts reflect from the screen and bounce back from the walls and wash out the dark parts. So, if you can, use dark matt colours where possible.
Gary.