Darker is better, but that means everything. Walls, ceiling, floors. Since painting the floors can be pretty messy, I suggest dark carpet.
Color matters, but not really. Flat paint is a REQUIREMENT!
That is, as the room gets darker, the reflections from that room will affect color on screen less and less and less. Deep blues, reds, greens, browns... they all do a great job of simply making the room into a proper theater.
I just finished my basement, and went with a burgundy color. Then dark grey for the ceiling with dark grey carpet. Some trim in the room and and doors behind the screen are a bright white, but have almost no impact on things. The doors are actually a bit distracting, so I may repaint them later (pain in the neck!).
Blackout shades/curtains on the windows (preferably both).
The more audio minded people will tell you that room acoustics is one of the single best investments you can make in a room to improve sound.
I can tell you that as a projector person - a can of dark paint or two is the single best investment you can make in any front projection setup after you get a proper projector and screen.
I would STRONGLY!!! recommend adding a lot of recessed/directional lights to the room. If the room is on a top floor or has retro access, adding 6+ recessed lights only takes about a day of your own labor to do. It is incredibly easy to do with most of the work being done from the room upward. A couple of zones of good lighting go a long way to make a dark room feel far less like a cave.
As to your wife... She gets every othe room in the house to be bright and cheerful in. A theater is purpose driven, and with proper lighting, even though the colors are dark, the room itself can be plenty bright when necessary with proper lighting. Include her in this, but help her to understand that dark colors don't mean depressing. Custom woodwork is often dark and covers entire walls of rooms. It is 'executive' looking, or 'upper class' to have some darker rooms. When you turn lights down, it helps you relax, and feel more comfortable. More homey. This is the type of feeling a dark theater helps to inspire, and they become very relaxing rooms instead of the bright high energy rooms in the rest of the home.