1. Preheat oven to 350°. Beat butter at medium speed with an electric mixer until creamy; gradually add sugar, beating well. Beat in egg whites.
2. Combine flour, baking powder, and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture. Stir in extract.
3. Divide batter into 3 bowls. Tint 1 bowl with desired amount red food coloring, and 1 bowl with desired amount blue food coloring. Layer blue, white, and red batter evenly into baking cups in 2 (12-cup) muffin pans. Bake 18 to 20 minutes. Cool in pans 10 minutes; transfer to wire racks to cool completely.
4. Frost cooled cupcakes with Fluffy White Frosting; garnish, if desired.