(snipped a bunch of things I tested)
Further bug-tracking hints: If I use a "Select All" before using the "Mask Brush" tool, then I can use it as stated (where I click it momentarily it shows the mask color and then it shows the usual lasso outline). And all other painting tools also work. If I use the Mask Brush on lassoed areas after inverting this "Select All" mask, then it erases whatever mask I had set with it. My Mask Brush is WORKING BACKWARDS!
Most strange.
... A day later and a few letters with Gerhard (my wasting his valuable time) ... and I figured out the problem.
As Homer Simpson would say, "D'oH!"
I read the help file again. I didn't stop to think that the Mask Brush would be dependent on the foreground painting color when using it. But just like painting on any mask, yes it is!
(Maybe I'm having a stroke.)If I use a white Mask Brush, then the brush behaves in reverse. If I use a black Mask Brush, then it behaves as how I was first learning to use it.
I'm going to leave this post up to help prevent other people from hitting their forehead and yelling "D'oH!"
What a nice mask-editing tool! NOW THAT I FIGURED OUT HOW TO USE IT PROPERLY.
(My apologies to Gerhard for wasting his time with my stupidity.
)