1-First Step: Prepare the Mise en Place Gather all tools: mixing bowls, whisk, spatula, six small bowls for colors. Measure dry ingredients: 4 cups flour, 2 cups sugar, 4 tsp baking powder, 1 tsp salt into one bowl. Soften 1 cup butter. Crack 4 eggs, measure 1.5 cups milk and 2 tsp vanilla. This setup keeps things smooth for busy cooks.

2-Second Step: Mix the Cake Batter Cream butter and sugar until light, about 3 minutes. Add eggs one at a time, beating well. Mix in vanilla. Combine dry mix alternately with milk, starting and ending with dry. Beat until just combined, no lumps. Batter yields about 6 cups total.

3-Third Step: Color the Batter Divide batter evenly into 6 bowls, roughly 1 cup each. Add gel color: red to first, orange second, yellow third, green fourth, blue fifth, purple sixth. Use 1/4 tsp gel per bowl, stir gently. Scrape sides for uniform color. Gel prevents bleeding unlike liquid dyes.

4-Fourth Step: Bake the Layers Spoon each color into prepared pans. Bake at 350°F for 15-20 minutes until toothpick clean. Rotate halfway if baking multiple. Cool in pans 10 minutes, then on racks 1-2 hours. Layers must be room temp or chilled to stack without sliding.

5-Fifth Step: Make the Buttercream Frosting Beat 2 cups softened butter 3 minutes until creamy. Add powdered sugar 2 cups at a time, mixing low. Pour 1/4 cup milk and 2 tsp vanilla, whip 2-3 minutes fluffy. Adjust milk for spreadable texture. Reserve 4-5 cups for assembly.

6-Sixth Step: Assemble the Cake Level cooled layers if domed. Stack on plate: purple bottom, blue, green, yellow, orange, red top. Spread 1/2 cup frosting between each. Crumb coat thin layer over whole cake, chill 30 minutes. Apply final thick coat, smooth sides. Pipe borders, add sprinkles.

7-Final Step: Finishing Touches and Serving Chill 20 minutes for neat edges. Slice with sharp knife for clean rainbow reveal. Serve at room temp. Total time 2-3 hours. Store as below. This pineapple sunshine cake fan will love this vibrant twist!