How to Make It
Preheat oven to 350°. Weigh flour or lightly spoon flour into dry measuring cups; level with a knife. Sift together flour, baking powder, and salt.
Beat granulated sugar and room-temperature butter at medium speed with an electric mixer until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Add flour mixture and sour cream; beat at low speed until just combined. Divide batter evenly among 3 bowls. Tint batter in one bowl with desired amount of red food coloring; tint batter in a second bowl with desired amount of blue food coloring. (Batter in third bowl remains untinted.)
Spread red, blue, and untinted batters into 3 separate 13- x 9-inch pans coated with baking spray. Bake 10 to 15 minutes or until a wooden pick inserted in the centers comes out clean. Cool cakes in pans on wire racks 10 minutes. Remove from pans to wire racks to cool completely.
Meanwhile, beat softened butter, milk, vanilla, and 4 cups powdered sugar at medium-low speed with an electric mixer until smooth. Add remaining 2 cups powdered sugar, 1 cup at a time, beating well after each addition.
Place cooled red cake layer on a plate; spread one-third of buttercream over top. Top with white cake layer; spread one-third of buttercream over top, and top with blue layer. Spread remaining one-third of buttercream over top of cake. Chill until buttercream sets up slightly; cut into cubes.