I showed a third image which is the result of mathematically subtracting the one image (converted) from the other (extracted}.
When John said he did not subtract, he also meant to subtract mathematically. That is to create a new image by subtracting each pixel value in one image from the same location pixel value in another image. That is to say that, if the pixel at x,y 100,200 = 50,60,80 in one image and 45,50,65 in the other, then the result of the subtraction is 5,10,15 ... a much darker image and indeed a black image** would mean no difference at all between the two compared images.
** if a result has a negative value, that value is set to zero, in which case black may not mean "no difference".
Not sure that question is relevant while we are talking about comparing two images by subtracting the one from the other.