Preheat the oven to 350F/177C convection (or 325F/162C conventional) and grease and line three 6 inch cake pans. 

In a medium bowl, whisk together the flour, baking powder and salt until they're evenly distributed.

Place the room temperature butter, the oil and the granulated sugar in a large bowl and use an electric mixer with the whisk attachment to beat them for 2 minutes. 

Add the eggs, one at a time, beating on medium speed until each egg is fully incorporated. Add the sour cream and vanilla extract and beat just until combined.

Add half of the dry ingredients to the wet ingredients, turn the mixer on low speed and then pour in the milk, followed by the rest of the dry ingredients. 

Distribute the batter evenly among the three cake pans, about 14oz in each pan and bake for 30-35 minutes. When the cake is fully baked it will release itself from the edge of the pan and it will spring back if you press on the center of it. 

Allow the cake to cool completely before frosting.

With the paddle attachment and on medium-high speed, beat the butter until it is pale and fluffy.

Lower the speed to low and slowly add in the powdered sugar, beating until fully combined.

Add in the vanilla extract and heavy whipping cream and beat until fully combined.

Once the cakes have fully cooled, place the first layer on a large plate or cake stand. Use an offset spatula to frost each layer, followed by a thin crumb coat.

Repeat the steps for vanilla buttercream to make the buttercream for the decoarations, and then separate the frosting into thirds.

Leave 1/3 of the frosting white and set it aside for emergency purposes (in case you color something too dark).

Take 1/3 of the frosting and color half of it pink and the other half blue.

Use the last 1/3 to make black. Mix in the black cocoa powder to the buttercream. The buttercream may get thicker so you can add in the last 1 Tbsp of heavy whipping cream here (start with 1/2 Tbsp and then add the rest if it's still not spreadable).

Add black gel food coloring, until the color is nice and dark (it may not be PERFECTLY black).

Place dabs of different color buttercream on the top and sides of the cake and use a cake spreader or an offset spatula to distribute the colors. 

If you add too much black, just add more of the lighter colors on top and vise versa. Just keep spreading it around the edge of the cake until you like how it looks. Remember that the more you spread it, the more the colors will blend together.

When your frosting is complete, take a small paint brush, dip it into your white gel food coloring and use your finger to spray it onto your cake.