1 pound (4 sticks) unsalted butter, at room temperature, plus more for greasing pan
6 cups all-purpose flour, plus more for dusting pan
2 teaspoons baking powder
1 1/2 teaspoons baking soda
1 teaspoon kosher salt
3 cups granulated sugar
1/2 cup firmly-packed light brown sugar
5 large eggs
2 tablespoons sour cream
1 1/4 cups buttermilk
1/4 cup grenadine
Finely grated zest of 2 lemons
Finely grated zest of 2 limes
Finely grated zest of 1 large orange