How to control brush color

Skiller Photoshopper here trying to get into Elements. One thing that I cannot get a handle on is how to switch brush colors, specifically when I'm in an adjustment layer with a mask. I want to work on the mask using black and white brushes, something I've done thousands of times in PS. Under COLOR on the left there's an icon with a pair of black andwhite squares with a little arrow suggesting you can switch between them, but no... that just opens a color picker window where I don't seem to have any control at all. If I choose a new color and click OK the new color does not even show on the icon. After much fumbling I did get a nice 50% gray brush to replace the white, but now I can't get the white back.

What am I missing? Any help will be much appreciated. Thanks.