Now is the difficult time. Don't blow it by letting her know what you are up to. You many want to slightly disagree with her on the color something benign like this:
But honey, what's your opinion on the pewter? Do you think it looks more bold (bold doesn't mean anything and won't offend her). After she makes some comment about how the white is better quickly agree like this (make it sound reluctant like you're giving in):
Well, you're probably right. I suppose given where we live and the fact that there really aren't that many white ones out there will make a pretty big difference. Then say: Well, let's get this done and get home. (this is the best because you can go either way with this comment. She could either balk at you like what do you mean "get it done" I'm not ready to buy this stupid thing! OR why don't you come back tomorrow on your own time and do the paperwork, I'm not very good at it. If you get the first response just say what you meant was let's get home, I wasn't saying let's just run in the office and sign the paper.
At all costs cover your ass and continue to make it her decision. Before you know you'll have your H2. I believe there is quite a kick as dealership there in St. Louis too!