I've only bought eye and lip makeup from MAC so I'm a total noob when it comes to the other products.

I just bought studio finish concealer last week from MAC. I've been using mineral foundation with a concealer brush this whole time and wanted something more concentrated. The MA helped me pick out the shade and I noticed that she chose a shade that was exactly my skin tone. Aren't concealers supposed to usually be a shade lighter to hide dark areas?

Also, what's the difference between NC and NW? I can't really tell from the pictures on the site.

Thanks