For the cupcakes: Preheat the oven to 350 degrees F. Line 2 standard 12-cup muffin tins with cupcake liners.
Place the cake mix, water, oil and eggs in the bowl of a stand mixer fitted with a paddle attachment and mix on low speed for 30 seconds. Increase the speed to medium for 2 minutes. Pour the batter evenly into the prepared muffin tins and bake until a toothpick inserted in the center comes out clean, 18 to 20 minutes. Cool completely on a wire rack.
For the buttercream: Beat the butter in the bowl of a stand mixer fitted with a paddle attachment on high speed until fluffy, about 2 minutes. With the mixer on low speed, gradually add the confectioners' sugar and milk, 1 tablespoon at a time, alternating between the two until no lumps remain. Add the vanilla and salt and mix until incorporated.
Divide the frosting evenly among 3 bowls. Stir the pink food coloring to desired tint into 1 bowl. Stir the purple food coloring to desired tint in another bowl. Leave the remaining bowl white.
Transfer half of the pink frosting to a piping bag fitted with a 2D tip. Use a small rubber spatula or small offset spatula to spread the frosting around the sides of the piping bag, leaving a hole in the center. Fill the middle with half of the white frosting with a 1-tablespoon ice cream scoop. Add the remaining pink frosting. (You may need to do this twice if all the frosting won't fit in 1 piping bag.)
Repeat the process with the purple frosting and remaining white frosting. Twist up both piping bags and tie them each off with rubber bands.
To make a rose, start in the center of the cupcake and make a swirl to the outer edge with the pink two-toned buttercream. Place a mint leaf on the edge of each cupcake.