Prepare icing and tint 1/2 cup blue and 3/4 cup red; reserve 3 1/4 cups white (thin 1 3/4 cups with 1 teaspoon light corn syrup). Ice cake sides smooth and lightly ice cake top with thinned white icing and spatula.
Using toothpick, mark 7/8 in. wide striped areas and blue field on cake top.
Pipe stars with tip 18: use red icing in red stripe areas, white icing in white stripe areas.
Fill in blue field with stars using tip 18 and blue icing. Use tip 18 and white icing to pipe stars on blue background.
Use tip 21 and white icing to pipe star bottom border.
The star tip creates the most celebrated, easily accomplished decorations! The serrated edges of the star tip make ridges in the icing as you squeeze it out. Because fill in requires so much piping from the same bag, keep replenishing icing in bag.