yeah, if its a big piece of food and its is red or blue, it could color the mouth or fingers a bit. This depends on how dry it is when being used, how long it is used (opposed to being gobbled up quickly), etc. But using little balls of cotton candy for things like grapes or just "food" on a plate to give the actors something to work with is easy to just put in the mouth without a mess, and even easier if using an eating utensil.
One thing I didnt mention was that you may have to play with it to get it right. Depending on your altitude, humidity, and temperature it may take longer or shorter to dry out properly and get the desired result.
I saw someone do this for a christmas carol and they used it for the goose legs. They used some plastic bones and covered it with the white cotton candy and let it harden and then gave it a light misting with some brown food coloring spray (not too much though, or it will dissolve the sugar. I think it got quite hard before they did this.) They then set one up on each side of the goose and the actor would grab one and take a bite and it would be brown on the outside and white on the inside. I was also told its very delicious goose!
I also heard they used to used this a lot in the 50's and 60's for movies. Even in Hello, Dolly, I think.
One thing to check before serving it to actors is to be sure that no one is diabetic or is allergic to artificial food coloring.
