Im making a cake and cupcakes for our pastor's birthday, Black and white music themed. I want to top the cupcakes with white buttercream with a black swirl. Ive researched techniques and I think Ill paint Americolor Super Black inside the pastry bag to give the frosting a nice rich black swirl. My HUGE concern is that when their guests eat them, they might get black stains from the dye on their lips/faces. Am I over-thinking this?? Or maybe its a possibility??
Also, you'll have to "restripe" the bag frequently to get a vibrant color. You'll be surprised how quickly you start "going gray". If you are doing a lot, your best bet it to use disposable bags, get them striped and use each bag for only one fill of buttercream.
Ask me how I know... c'mon ask me.
275 cupcakes with blue and yellow tipped swirls for the Cub Scout Blue & Gold banquet. Never again.
I would not stripe the bag, that will give it the swirl effect, if you want the rich black color, tint your BC with the Americolor super black and pipe it on that way. You don't need alot, it will get darker as it sits longer, and yes, warn your guest, it may turn the teeth and tongue black....... 
thanks everyone!! Im only making 12 or so as a "bonus"... Im making a 9 inch cake for them, and theyre expecting 20 people. Wanting to make sure there was enough cake/cupcakes to go around.
Im making fondant music notes to go on top... i just really want a two toned swirl of frosting on each cupcake. I'll be sure to warn everyone about possible stains. 
Is there any other way of achieving this look WITHOUT the possibility of stains??
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