Preheat the oven to 350°F (175°C). Grease and flour three 9-inch round cake pans.
In a large mixing bowl, cream together the sugar and butter until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla.
In another bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add to the creamed mixture alternately with the buttermilk.
Divide the batter evenly into six bowls. Add a different food coloring gel to each bowl and mix well. Pour each color into the prepared pans, layering as desired to create a swirl effect.
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Allow to cool before frosting.
Notes
For best results, use gel food coloring as it tends to give more vibrant hues than liquid colorings.