For me personally, the best way to achieve bright white icing is to make sure everything going into the icing is either bright white or clear. I use all clear extracts and the confectioners sugar and meringue power I use are both bright white. I will occasionally add roughly 4-5 drops of white food coloring or even a drop or two of blue coloring if I am matching it with other colors that have a slight blue hue. This would be for a batch size calling for 2 lbs of confectioners sugar.