Tell me how she changed dresses with her walking and only part of body her shielded and her dress changes in front of our eyes
OK, so anyone unhappy with spoiler here can blame Steve.
Here is a frozen image of that transition. The dress is going from green color to purple as she holds the hoop with shredded fabric:
Look at the gap at the bottom where the dress shows through over her leg. What do you see? Yup, some of both colors/dresses!
Or when he throws the confetti over her and her dress changes instantly
Too lazy to capture that frame and upload it but if you pause the video, you notice that for a good 1 or 2 seconds, she is completely covered with confetti -- plenty of time to undo the top layer to expose the dress color beneath. In full speed, it seems as if it never covers her fully but in slow mo, it definitely does.