• Preheat oven to 350℉. Gather as many 8-inch round cake pans as you can find. Butter and line the cake rounds with parchment paper rounds. Set aside.

  NOTE: You may bake the layers in batches if you don't have enough cake pans. Clean cake pans between baking.
• In a bowl of a stand mixer with the paddle attachment, cream butter. Add sugar and continue to cream until lighter in color. Slowly add egg whites to the butter on low-medium speed. Add vanilla and sour cream. Mix until thoroughly combined.
• In a medium bowl, whisk together flour, baking powder, baking soda and salt. Alternate adding flour mixture and milk to the creamed mixture. Add the flour in three additions, beginning and ending with the flour.
• Use a kitchen scale to weight cake batter. Take the weight and divide it by 6. (My total weight was 1530 grams. Divide by 6, it was 255 grams.) Measure out equal amounts of batter into 6 small bowls.

  Note: If you don’t have a scale, it was roughly 3/4 cup of batter into 6 bowls.
• Add food coloring to individual bowls. I used about 4-8 drops of coloring for each. Some colors required more gel than others to get the vibrant hues I wanted. Be sure to use food gels. Adding liquid food coloring will make your batter too runny and thin.
• Transfer colored batter to prepared cake pan. Bake for 13-15 minutes until cake is set. Let cool in pan for 3 minutes before unmolding and allowing to cool completely on the racks. Continue to bake off cakes until you have completed all six colors.

Buttercream:

• Whisk together egg whites and sugar in the bowl of stand mixer. Place bowl over a pot of simmering water to create a bain-marie or double boiler. Make sure the simmering water does not touch the bottom of the bowl. Whisk mixture until it reaches a temperature of 160°F.

  *Alternatively, if you don't have a thermometer, heat the mixture until all the sugar has melted. Test this by dipping your pointer finger into the mixture and rubbing it against your thumb. The mixture should feel smooth. You should not be able to feel the granules of sugar.
• Return bowl to stand mixer. Using the whisk attachment, beat mixture at medium speed until it has cooled to room temperature (about 75-80°F) and meringue has started to form.
• Reduce to low speed. Add butter one tablespoon at a time. Add salt and vanilla. Increase speed to medium-high and continue to beat until buttercream is light and fluffy.

Assembly:

• Place the purple layer of cake on a cake board. If desired, lightly soak cake layer with simple syrup (or other flavored syrup). Spread about 1/4 cup buttercream on cake.
• Repeat adding syrup and buttercream on remaining layers (blue, green, yellow, orange, and red).
• Spread a thin layer of buttercream around the sides of cake to seal in the crumbs (crumb coat). Place in the fridge and allow to chill for 15 minutes to set buttercream.
• Coat cake with final thicker layer of buttercream. Decorate cake as desired. Store cake in the fridge until ready to serve.