To make the spectacular bursts on July 4th desserts, you'll need three pastry bags—one for each color. Fit one bag with a small round tip (we used Ateco #2) and the remaining two with very small tips (such as Ateco #0).
Whisk water and meringue powder with a mixer on medium speed until soft peaks form, about 3 minutes. Add sugar, and beat until icing holds a ribbonlike trail on the surface for 5 seconds when beater is raised. (Icing should be the consistency of glue.)
Transfer two-thirds of the white icing to the squeeze bottle with the larger hole or to the pastry bag fitted with the larger tip.
Divide remaining icing in half, and tint one half using red food coloring and the other half using the 2 blue food colorings until desired colors are achieved.
Transfer red and blue icings to the remaining 2 squeeze bottles or the pastry bags fitted with the smaller tips.
Icing can be refrigerated in an airtight container (with a damp paper towel covering the surface) for up to 1 week.