C# image subtraction

Question

I am using the attached project code which i came across on stackoverflow in order to compare a known image and then provided with the diff image i am wanting to keep the deltas and remove any matched pixels or at least set them to white or other colour?

 

So far with the 2 images i am using (orig.jpg, diff.jpg) i end up with an exact clone of the diff image and nothing removed? I would like to see just the new object in diff.jpg in the final image; has anyone achieved this and know a good way to get this working even if its via use of paid libraries?

Many thanks