Hello,
(I am a cartoonist. Sorry, I'm French and don't write well English.)

I have a new little idea of ​​Krita's functions.

Sometimes I draw my comics with two colors. Black for drawing and another color for shadows.
There are many designers who do the same thing.

With an eraser that don’t erase black, we could work on a single layer.
(I tried to adjust a «brush/eraser” for that. But I did not succeed.)

So we would draw without wondering if we selected the right layer.

It should be possible to create a special blending mode that would do this, as well as a blending mode that can overpaint anything but black, and name them, say, cartoon-normal and cartoon-erase.