When you have a night sky that takes up a significant portion of a city scene, adding fireworks can imbue the picture with excitement and visual drama. That's what I did to this shot of the famous Chinese Grauman's Theater in Hollywood, California. To add the fireworks, it's not necessary to separate each streak of color from the background. Rather, I made a rough selection around an original burst using the lasso tool, and then I used Edit > copy. Now the image was in the clipboard, Photoshop's temporary holding place for one photo or a part of a photo. Then I used Edit > paste to place the fireworks into the night sky above the architecture. The black color behind the fireworks in the original picture blended with the black sky above the theater, and the composite therefore looks perfectly believable. I resized the bursts as i added them using Edit > transform > scale. I added three different fireworks shots to complete this image.
This is one of the techniques I demonstrate in the Photosho seminar I teach in my home. The next one is in December.