Preheat your oven to 350°F (175°C). Grease and flour a 10-inch Bundt pan.

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

In a large mixing bowl, cream the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with the milk. Begin and end with the flour mixture. Mix until just combined.

Divide the batter into three equal portions. In the first bowl, mix in the pureed strawberries and red food coloring if desired. In the second bowl, mix in the pureed blueberries and blue food coloring if desired. Leave the third portion plain.

Pour the red batter into the prepared Bundt pan, followed by the plain batter, and then the blue batter. Use a knife to gently swirl the colors together for a marbled effect.

Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for 15 minutes before inverting onto a wire rack to cool completely.

Once cooled, dust the top with powdered sugar before serving.