Using Paint version 6.1 (Build 7601: Service Pack 1), Windows 7
Steps to reproduce
open paint
Save As monochrome bitmap (.BMP)
Click on Pen to draw lines (Left click to draw black, right click to draw white)
Drawing white does not work.
Found a work around by changing the properties to color, doing the edits, then changing back to monochrome before saving.
But this is not a preferred solution and not appealing to an end customer, my preference is to fix the issue.
Thanks