Did you ever eat icing that contained so much food coloring it tasted bitter? This is usually due to dark icing having so much food coloring in it. I have a solution though, instead of adding the color to white buttercream add it to chocolate buttercream. This will prevent that bitter taste that usually comes with adding a lot of food coloring. By starting off with chocolate you have a nice flavor and a dark color, therefore you will need less color so less bitterness. Just remember to tell your customers you are now using chocolate icing just in case they have any chocolate allergies!