my black icing started bleeding on to the cake. if i put it on the white icing, it will turn the icing purple. Has this happened to anyone else? Do you know how to prevent it?

AYou are putting too much color in it. Just put enough in to be a dark grey and it will darken the rest of the way. . Also, I find Americolor black to be the best. I recently misplaced mine (it was just hiding behind a bigger 16 oz bottle) and use Chef Master coal black, and it turned greenish. So I added brown to try to cancel it and it turned iridescent like a beetle. I found the Americolor after that!