I have edited with Ctrl+left mouse click a small area of pixels (see the snippet below) which yielded the small dark rectangle in the l.h.s. half of the blue bottom picture. Now I wanna to set this to white color. How shall I proceed ? I have also added the "Mask.001" node but it seems to have no effect to the output. (Perhaps I have weight 1 on the other layer picture and I have to change this somehow).