+1 to going with an actual paint shop and having them mix you up a batch.
Although I did find, when I painted some wind deflectors to go in front of my grips in the winter, that Rustoleum has a silver that matches the VN900 silver really stinking good. It's a silver metallic, and my eyes can hardly tell the difference, though of course the quality of the paint is not quite as good as real automotive paint.
The best solution though is, again, having a paint shop mix it up. Colorrite is not expensive because they make good paint, they are expensive because they offer it up pre-packaged and shipped to your door. The paint shop guy will be able to mix it up to a perfect match for much much less. Sometimes even places like Sherwin-Williams or the local hardware store can do it. I worked at a Hardware store and mixed paint, and although we didn't stock it, we could order a couple different brands of automotive paint and mix it up.
As far as a primer, I'm not a pro painter or anything, but I would think a good 'scuffing up' would suffice. Though the primer couldn't hurt. If you end up going to the paint shop to get some mixed up, ask them. They'll be more knowledgable AND be able to tell you what works and what doesn't for whatever brand they mix. Some paint can be shot over that black paint really easy, other paint won't adhere well and may develop lots of orange peel if not shot over a specific type, color, or brand of primer!
Oh, and if you were planning on doing this as a rattle-can job, the paint shops can do that too. Several manufacturers sell 'tintable' aerosol paint cans that the paint shop can prepare for you. It's a great way to do touch up paint or paint small parts (or even a whole bike if you've got the know how! Lots of pro's spray-paint parts even on show cars, but of course that's with a great amount of knowledge and skill, and not with your typical $1.99 wal-mart spray paint either)
"8 But God demonstrates his own love for us in this: While we were still sinners, Christ died for us."
Romans 5:8 (NIV)
2006 Vulcan 900 Classic LT "Couch-a-Saki/Mini-Bagger"
2011 Honda Shadow Aero 750 (Wife's)