Preheat oven to 350 degrees and line a muffin pan with cupcake liners. In a large bowl, mix together the butter, eggs, milk and yogurt. Add dry cake mix and blend until smooth.
Fill each cupcake liner 3/4 full. Bake 17-20 minutes, until a toothpick inserted in the center comes out clean. Cool completely on a wire rack.
To make the frosting, blend the butter, yogurt and dry frosting mix in a large bowl until creamy. Divide frosting evenly into three small bowls then use red and blue food coloring to color frosting, leaving one bowl white. Mix thoroughly to incorporate.
Place a piping bag or plastic ziplock bag into a glass, folding the ends of the bag over the edge. Use a spatula to place each color of frosting into the bag, twist the ends closed then use your hands and gently squeeze the bag of frosting to mix the colors.
Cut the tip of the piping bag with a scissors and pipe the colored frosting onto each cupcake.