Recently I discovered a way of coloring my children's cakes with these natural food coloring's from Hopper's Natural Foods.
They have a great range of colors and even sprinkles and 100's of 1,000's. You can order them online here and even though they are based in Australia they do overseas orders as well.
They are made from natural ingredients for example the orange is made from carrots funnily enough.
Just a word of advice, it is suggested to add citric acid to help brighten the colors - just don't add as much as me or your icing will be very lemony!! Check out my daughters birthday cake colored with Hoppers!