To reproduce:
1. Run the app. (Before runnung the app put attached image to the project dir)
2. Drag from source to destination.
3. Open Mac Digital Color Meter and check the difference between color in J letter for example.
Another way is to click "Screen shot" button
and check images with prefix AWT (in project dir in img folder),
because
images made by standard javafx means don't differ.
This behavior happens only when dragged image is rendered on a separate stage.
When the image is rendered after dnd on the same stage as the original it doesn't change color.