brush tool and masking -- what are the rules for foreground/background color?

I often use the brush tool for masking layers. I use the backslash (\) to show what the mask looks like, and I paint with the brush in both the mask-visible and the mask-invisible states. Sometimes I paint with a white brush - taking away masking -- and sometimes I paint with a black brush, adding masking.

I am always surprised by what color the brush is when I click on the backslash key. Sometimes it stays black, sometimes it stays white, and sometimes it changes.

What are the rules that govern the foreground/background color of the brush when using the backslash key on a layer?

Thank you,

Phil