Special equipment: 24 picket fence cupcake liners (available online or baking supply stores) 4 pastry bags, or resealable plastic bags with 1 corner snipped Large star pastry tip (#885 or similar) 4 tier (23 count) cupcake tree (available online home stores)
Preheat the oven to 350 degrees F. Line 2 (12 cup) muffin tins with white paper liners.
Make the cake mix according to package instructions, substituting the milk for water. Pour the batter into the lined muffin tins and bake until golden and a toothpick inserted in the center comes out clean, about 12 to15 minutes. Remove the tins from the oven and let them cool completely on a wire rack.
In a large bowl, using a hand mixer, beat the frosting with enough confectioners' sugar to create a stiff consistency. Divide the frosting among 4 bowls. Color 1 with blue, 1 with red, and 1 with green, leaving 1 white.
When the cupcakes are cool, drop each into a picket fence liner.
Fit 4 pastry bags with large star tips and fill each with a different color. Pipe a swirl of frosting onto 6 cupcakes with 1 color. Repeat with remaining cupcakes and colors. Arrange them on the cupcake tree, alternating colors and top with nonpareil sprinkles.