Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
In a large bowl, cream together butter and sugar until light and fluffy. Add eggs, one at a time, mixing well after each addition.
Mix in vanilla extract. In a separate bowl, whisk together flour, baking powder, and salt.
Gradually add dry ingredients to wet mixture, alternating with milk, beginning and ending with dry ingredients. Divide batter evenly into three bowls.
Color each batter portion with food coloring: red in one, blue in another, and leave the third uncolored.
Pour spoonfuls of each batter into prepared pans, layering to create stripes. Bake for 45 minutes or until a toothpick inserted in the center comes out clean.
Allow cakes to cool in pans for 10 minutes, then turn out onto wire racks to cool completely.
Assemble by stacking layers, frosting as desired.
Notes
For best results, chill the cake before serving to enhance the stripe appearance.