Edited picture-setting to new color 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 &quot;Mask.001&quot;
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).

EDIT

EDIT2
The issue is that your mask is not the correct size for your image, and you need to change the Mix Node settings from &quot;Color&quot; to &quot;Mix&quot;.
You need to change the settings for your mask node to &quot;Fixed&quot; instead of &quot;Scene Size.&quot;  Then, type in the x/y dimensions of your image.  I also changed the mix node to &quot;Mix&quot; instead of &quot;Color.&quot;

If you don't know how to find the dimension of your image, open it on Windows Explorer, right-click on it, and press &quot;Properties.&quot;  You can find the dimensions under the &quot;Details&quot; tab.
Hope that fixes your problem!  Let me know if you have any other issues.