1. Prepare the Natural Dyes

• Juice beets in a juicer or use the liquid from canned beets.
• Juice carrots in a juicer or use store-bought carrot juice.
• Juice spinach in a juicer.
• Microwave 1/4 cup (25 g) frozen blueberries in 30-second intervals until they burst, then strain out 1+ tablespoon (15+ mL) blueberry juice.
• Microwave 1/4 cup (25 g) frozen blackberries in 30-second intervals until they burst, then strain out 1+ tablespoon (15+ mL) blackberry juice.

2. Prepare the Cake Batter

• Preheat oven to 350°F (175°C).
• Cream the butter, oil, and sugar in a large mixing bowl until light and fluffy.
• Add the egg whites and beat well.
• Mix in vanilla, 1 cup (237 mL) milk, and yogurt.
• In a separate bowl, whisk together the flour, baking powder, and baking soda.
• Gradually add the dry ingredients to the wet ingredients, alternating with the remaining milk.

3. Color the Cake Batter

• Divide the cake batter into six equal portions (about 1/2 cup per bowl).
• Mix in the natural dye for each bowl:
  • Red: 2 tablespoons (30 mL) beet juice
  • Orange: 1 1/2 tablespoons (20 mL) carrot juice
  • Yellow: 1 egg yolk + 1 tablespoon (15 mL) milk
  • Green: 1 tablespoon (15 mL) spinach juice
  • Blue: 1 tablespoon (15 mL) blueberry juice
  • Purple: 1 tablespoon (15 mL) blackberry juice
• If needed, adjust the color intensity by adding more juice.

4. Bake the Cake Layers

• Grease and flour six 5.5-inch cake pans (or bake in batches).
• Pour each colored batter into a separate pan.
• Bake for 10-15 minutes, or until the cake springs back when touched and a toothpick inserted in the center comes out clean.
• Cool for 5 minutes in the pan, then invert onto wire racks to cool completely.

5. Make the Buttercream Frosting

• In a large bowl, beat butter, powdered sugar, vanilla, and milk until smooth.
• Adjust the consistency by adding more powdered sugar (for thickness) or milk (for softness).

6. Make the Whipped Cream Frosting

• Beat heavy cream, powdered sugar, and salt at medium speed until stiff peaks form.
• Fold in vanilla extract.

7. Assemble the Cake

• Place the purple layer on a serving platter.
• Spread 2 spoonfuls of buttercream frosting evenly on top.
• Repeat with the blue, green, yellow, orange, and red layers.
• Lightly freeze the cake for 5 minutes to set the frosting.
• Coat the entire cake with buttercream frosting using a knife or spatula.
• Apply whipped cream frosting for a smooth, soft finish.
• Serve immediately.