Rainbow Layer Cake

Preheat your oven to 350°F (175°C) and lightly grease and flour your baking pans, or spray with non-stick baking spray.
In a large bowl or the bowl of your stand mixer measure out the dry ingredients (all-purpose flour, sugar, baking powder, and salt). Whisk or mix to distribute the flour. Form a well in the center of your dry mixture.
Pour the milk, melted and cooled butter, eggs, and vanilla extract into the center of the dry ingredients then mix at medium speed for 2 minutes until smooth.
Divide the batter into either 5-6 equal portions in small bowls and add gel food coloring to create your desired colors. Transfer the batter into your prepared baking pans.
Bake on the middle of your oven's center rack at 350°F (175°C) for 18-20 minutes or until an inserted toothpick, knife, or cake tester comes out clean.
Remove from the oven when done and allow the cakes to cool in the pans for 5 minutes before transferring to a wire cooling rack to cool completely.

Once the cakes are cooled, mix your buttercream frosting and level each of the cooled cakes. Spread a thin spot of buttercream on the base of your cake plate or your cake board and start layering the cakes.
Pipe frosting between each color cake layer as you stack them 5-6 high then apply your crumb coat to the top and sides of the layered cake. The crumb coat should be approximately ¼-inch thick. Chill in the refrigerator to set the crumb coat.
Portion out 5-6 equal amounts of your frosting, depending on how many cake layers you have. Color the frosting to match your cake layers.
Pipe or spread the colored frosting on the matching cake layers and then in concentric circles starting in the center of the top of the cake. Smooth and blend the frosting colors together. Pipe a bottom border if desired.
Top the cake with matching mini jelly bean colors that are arranged on the top of the cake in circles, to match the frosting.