My brush tool won't paint in white

[#1]

My brush tool won't paint when the color is set to white.  It paints in all other colors, but not in white.  Which means when I am using a black mask, I can't remove portions of it.  

[#2]

My brush tool won't paint when the color is set to white.  It paints in all other colors, but not in white.  Which means when I am using a black mask, I can't remove portions of it.