Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

In a separate large bowl, cream together the butter and sugar until light and fluffy.

Beat in the eggs one at a time, then stir in the vanilla extract.

Gradually mix in the dry ingredients, alternating with the buttermilk, until just combined.

Divide the batter into two equal portions and stir in the red food coloring into one portion and the blue food coloring into the other.

Pour the colored batters into the prepared baking dish, alternating colors to create a flag design.

Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.

Allow the cake to cool completely.

In a chilled bowl, whip the heavy cream, powdered sugar, and vanilla extract until stiff peaks form.

Frost the cooled cake with the whipped cream and decorate the top with strawberries, blueberries, and raspberries in a flag pattern.

Refrigerate the cake until ready to serve.