Rainbow Bundt Cake

CAKE:

 * 3 cups all purpose flour
 * 2 teaspoons baking powder
 * 1 teaspoon salt
 * 1/2 teaspoon baking soda
 * 2 1/4 cups granulated white sugar
 * 1 cup (2 sticks) unsalted butter, at room temperature
 * 1/4 cup vegetable oil
 * 4 large eggs, at room temperature
 * 1 tablespoon vanilla extract
 * 1 1/3 cups buttermilk
 * red, orange, yellow, green, blue and purple food coloring (gel paste works best- see notes below)

LEMON GLAZE:

 * 3 cups powdered sugar
 * 1 teaspoon finely grated lemon zest
 * 4 to 5 tablespoons freshly squeezed lemon juice

PREPARE THE CAKE:

 * Preheat the oven to 350 degrees F.  Spray a 12-cup bundt pan with nonstick spray.
 * In a medium bowl, whisk together the flour, baking powder, salt and baking soda.
 * In a separate larger bowl, use a hand mixer to combine the sugar, butter and oil on medium speed for 4 minutes, or until light and fluffy.  Beat in the eggs, one at a time.  Add the vanilla and beat for 2 more minutes.
 * Alternately beat in the flour mixture and buttermilk, making three additions or flour and two of buttermilk, and beating until incorporated.
 * Divide the batter equally among 6 small bowls, about 1 1/3 cups in each bowl.  You can leave one portion in the mixing bowl.  Stir in 4 to 8 drops red food coloring into one bowl and stir until thoroughly blended and the color is even (see Recipe Notes for color tips).  Repeat with other food coloring and batters.
 * Transfer the red batter to the prepared pan, spreading evenly using a spoon or an offset spatula.,  Repeat the process with the remaining batters in the following order: orange, yellow, green, blue and purple.  Do your best to layer each color evenly without swirling into other colors (expect imperfections).  The batter requires no swirling, since the batters will magically swirl on their own during the baking process.
 * Bake for 55 to 65 minutes or until puffed and golden brown and a tester inserted in the center comes out clean.  Let cool in pan for 10 minutes, then carefully invert the cake onto a wire rack to cool completely.

PREPARE THE GLAZE:

 * In a medium bowl, whisk together the sugar, zest and 4 tablespoons juice until smooth.  Whisk in more lemon juice if necessary (1 teaspoon at a time) until the glaze is thick, but pourable.
 * Pour the glaze over the cooled cake, letting it drip down the sides.  Let the glaze set for at least 15 minutes before serving.