Bug 457530

The current behavior of how masks are affected by color isn't intuitive to artists at all. 

At the most basic level: why does painting white on a mask *add* more of the visual effect, despite erasing doing the opposite by removing the visual effect? Some examples:

* I have finished inking something and I want to add screentone. I lasso select a piece of the artwork and create the screentone fill layer. However, it turns out my selection wasn't perfect, so I use my eraser to remove bits I don't want. Going by intuition, it would follow that going back to a black opaque color would add bits that I do want. However, this turns out to not be the case: painting in black on the mask will instead remove more the halftone. I must switch to white if I want to add additional halftone.

* There are bits of my artwork that I want to hide in a transparency mask. I select the bits I want to hide, invert, then apply a transparency mask. Like before, it seems like my selection was not perfect, so to resolve this matter, I use my eraser to hide parts that I don't want to seen, and *attempt* to use my black brush to show what I do want to be seen. As shown in the previous example, this is apparently foolish. (I also find it unintuitive that applying transparency masks applies the transparency to everything outside of the selection, but that's a bit of a separate matter.)

I get caught up in this a lot, so it would be really nice if this made more sense. Erasers and white color should both have the same removal-like effect on a mask while black should have an additive effect.