Erasable/Instant Dry Paint
Why have a great painting go bad, right when you put the last touches to it, when you can just erase the mistake and paint it back in the right way?
Just use erasable instant dry paint. Just like erasable ink but in paint form. You can add a lacquer or varnish to make the paint permanent. I know most painters of canvas paintings hate to cover up some large mistake with white paint which then ruins the picture by making the picture bumpy in places. Canvas and paint is expensive to replace. Maybe we could use reloadable paint brushes with paint colors you mix your self. I don't know about the brushes but, erasable paint sounds more logical than waiting for hours while your mistakes dry.