After you paint with the Adjustment Brush you can hover your mouse over the pin (that's the little dot the Adjustment Brush creates on your photo) and Camera Raw will show you a white overlay indicating where you've brushed on the photo. I find it helpful to see if I've missed any spots while painting. But what if you're painting over something that's white? It'll be pretty hard to see the overlay and tell if you did miss any areas. Well, all the way toward the bottom of the panel right next to the Show Mask checkbox you'll see a little rectangular swatch. If you click on it, you'll open a Color Picker dialog and you're free to change that overlay to any color you want.