Preheat oven to 350F. Grease 10 cups from a 12 cup muffin tin (or line with paper cups).

In a medium bowl, whisk together flour, baking powder and salt.

In a large bowl, whisk together sugar, eggs, vegetable oil, buttermilk and vanilla extract. Pour in dry ingredients and stir until just combined.

Divide batter evenly into 5 small bowls; each should have a little more than 1/3 cup batter (approx 6 tbsp or so for each).

Add about 1/2 tsp food coloring to each bowl to make red, orange, yellow, green and blue batters. Stir well, so no streaks of plain batter remain. Add additional food coloring if necessary.

Starting with the blue batter, add a small spoonful to each of the 10 grease muffin cups (just over 1/2 tbsp in each). Repeat with all remaining colors, working from green to yellow to orange to red, adding each subsequent spoonful on top of the previous color. Do not attempt to spread the layers of color out (as it can cause layers to combined), but allow them to spread on their own.

Bake for about 15 minutes, until a toothpick inserted into the center comes out clean.