But purple??! Yikes! You want to convince your girlfriend (which, why? Girlfriend? WTF cares what a girlfriend thinks???!) but if you do want to convince her its real easy. You get yourself some purple fabric, foot or two square, and another one a nice neutral gray like you want for your walls. Stand her in front of a mirror. Have her look at her face with the nice neutral gray draped across her shoulders. Then try the purple. While she is looking at how ghastly she looks you say that’s how all our movies are gonna look if we paint the walls purple.
Dark purple will be absolutely hideously awful. So awful you will probably break up with your girlfriend over it, then be stuck having to repaint, and when you do remember two coats, primer first to block out the hideous purple then your lovely neutral gray over the primer.
Also by the way in case you are wondering I went through the same and when the room was painted wall to wall and ceiling a nice neutral gray I was surprised how many people comment on how nice it looks. Not quite battleship gray, there is a hint of color to it but only a hint hard to pin down. Dark enough to keep the whole room from lighting up and washing out the contrast on the brighter scenes, light enough not to suck up all the light when the lights are on. Perfect. Totally sends the message this is a special room. Colors are a lame attempt at setting the mood. You don't want that. The whole idea here is the movie sets the mood. Neutral gray supports that message. Best thing you can do.