Preheat oven to 350 degrees, and fill a muffin pan with clear cupcake liners.
Add butter to bowl of standing mixer and beat on medium high speed. Slowly pour in granulated sugar and continue to mix until light, fluffy and well combined.
Add in eggs, one at a time.
Turn mixer down to slowest speed. Carefully add in cake flour, baking powder and salt then add in milk, vanilla and oil until just combined. Don't overmix!
Turn off mixer and remove half of batter, placing in a separate bowl.
Tint each bowl of batter with separate food colorings until you get the color you desire.
Alternate adding teaspoons of each color batter into cupcake liners until liners are 2/3s full (don't overfill these since they rise high).
Bake for 16-19 minutes or until a toothpick inserted into the center comes out clean.
Cool for 10 minutes then remove cupcakes from pan to cool completely to room temperature.