> Interesting results.  I will try to understand and October is going to be "look into Overlays"!  Will use your attached files.

I am not sure if it is correct to do the tests with the
test_subtract.xml project I provided. I used 2 png images of FullHD
size and then resized them in the Compositor via Projector.
More correct is to use the other 2 images I attach where the colored
squares are already the correct size and everything else should be
transparent background. I also attach the test_subtract_2.xml project
that uses these 2 new images.
Trying the overlays with these new images show that in the blend they
also use the white values we find instead of the transparencies.