Should I expect any surprise when trying to shoot green fireballs like this?
I would not expect boric acid to cause ignition of nitrocellulose. However, boric acid may react with un-nitrated hydroxyl groups on the cellulose backbone--likely a benign reaction. You might need more than a few drops of boric acid solution to see a green color - perhaps immersion in boric acid solution will be needed. Copper salts might give a good green color, so that is also worth trying. Either way, you will want to let the nitrocellulose dry thoroughly before trying to ignite it. 

Also, be sure to carefully practice the words: "I AM OZ, THE GREAT AND POWERFUL" before you go public.