Preheat oven to 350º F (180º C). Generously, spray 12-cup fluted tube cake pan with baking spray with flour.
Combine milk and vinegar in a small bowl. Let the mixture stand at room temperature for 5 to 10 minutes. When it is ready, the milk will be slightly thickened and you will see small curdled bits. Set aside.
In a large bowl, beat the sugar and butter until creamy and pale. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add vanilla and mix to combine.
Add the flour alternately with the milk mixture to the creamed butter, beginning and ending with the flour. Scrape down the mixing bowl with a spatula and turn the batter over several times to ensure all of the ingredients are well incorporated. Beat just until incorporated. Do not overmix this batter.
In a small bowl, place 2 cups of the batter; stir in 1 teaspoon of the red food color until blended.
In another small bowl, place 2 cups of the batter; stir in 1 teaspoon of the blue food color until blended.
Pour red cake batter into the bottom of the prepared fluted cake pan. Carefully pour the remaining white batter over red batter in the pan. Carefully pour blue batter over white batter. Note: Blue batter does not need to cover white batter completely.
Bake for 40 - 45 minutes, or until a skewer comes out clean when poked in the center.
Remove from the oven and allow the cake to cool for 10 -1 5 minutes. Remove from the pan; let the cake cool completely before decorating.