Preheat oven to 175°C (350°F). Line cake pans with parchment paper and lightly grease.

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

In a large bowl, cream the butter and granulated sugar with an electric mixer until light and fluffy, about 3 minutes.

Beat in eggs, one at a time, mixing well after each addition. Add vanilla extract and mix in.

Add the flour mixture in three parts, alternating with milk, starting and ending with flour. Mix until just combined.

Divide batter evenly into six small bowls. Tint each bowl with a different food coloring shade (red, orange, yellow, green, blue, purple).

Pour each colored batter into its own prepared pan and spread evenly.

Bake each layer for 15 minutes, or until a toothpick comes out clean.

Let layers cool in pans for 10 minutes, then remove and cool completely on wire racks.

Make the frosting: Beat butter until creamy. Gradually add powdered sugar, vanilla, salt, and milk until light and fluffy.

Level cooled cake layers if necessary. Place the purple layer on a serving plate and spread a thin layer of frosting on top.

Stack remaining layers (blue, green, yellow, orange, red) on top, spreading frosting between each layer.

Frost the top and sides of the cake with remaining buttercream. Decorate as desired.