And something that came to mind (Or perhaps is already possible --but unknown to me..) is invisible paint.
I know colour as pigment is separate from the material/paint. However I don't know of any way to erase only just colour --In the sense that you would end up with alpha and just the material/paint. So it would perhaps only give itself away by some specularity, and obviously as soon as you do end up painting there.
So a kind of alpha brush or colour only remover.. Seems like an interesting thing to me.. I can't even imagine the new ground.
But suppose I hope it's already there by some method I didn't find yet, hehe.

^^