Prepare the Batter: Preheat your oven to 350°F (175°C). Grease and flour six 8-inch round cake pans. In a bowl, whisk together flour, baking powder, and salt. In a large bowl, cream the butter and granulated sugar together until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract. Gradually add the flour mixture and milk alternately to the creamed mixture, starting and ending with the flour mixture.

Divide and Color the Batter: Divide the batter evenly into six separate bowls. Add a different gel food coloring to each bowl to create red, orange, yellow, green, blue, and purple batter. Pour each colored batter into the prepared cake pans.

Bake the Layers: Bake in preheated oven for 15 to 20 minutes or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then turn them out onto wire racks to cool completely.

Prepare the Whipped Cream Frosting: In a large mixing bowl, whip the heavy cream, powdered sugar, and vanilla extract until stiff peaks form.

Assemble the Cake: Place the purple layer on a serving plate and spread a thin layer of whipped cream frosting over the top. Repeat the process with blue, green, yellow, orange, and red layers. Spread more whipped cream frosting over the top and sides of the entire cake.

Decorate: Top the cake with colorful sprinkles and any edible decorations you desire.