Heat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
Mix cake mix, water, oil, and eggs in a large bowl. Beat for 2 minutes until smooth.
Fill liners ¾ full. Bake 18–22 minutes, or until a toothpick comes out clean. Cool completely.
Core the center of each cupcake. Fill with 1–2 teaspoons lemon curd. Replace tops if desired.
Beat butter until fluffy. Add powdered sugar gradually, then lemon juice and zest. Mix until creamy.
Divide frosting in two. Tint one pink, one yellow with gel coloring.
Fill piping bag with both colors. Pipe swirls on cupcakes. Top with sprinkles, zest, or mini lemon slices.