Achieve the brightest white in your icing using The Sugar Art's Master Elite White food color.  Master Elites are highly intense powder food colors.  Intended to color royal icing, fondant, buttercream, macaron batter, cake batter , and more!
Pro Tip: When using Master Elites Black and White, it will take more color to achieve the truest black and white tones.  This is why they are only sold in large jars.