Heat oven to 350°F (325°F for dark or nonstick pans). Place white paper baking cup in each of 24 regular-size muffin cups.

Make cake batter as directed on box for cupcakes, using water, oil and eggs. Divide batter evenly among 6 medium bowls. Add a different food color to each bowl to make red, orange, yellow, green, blue and purple. Place 1 level teaspoon of each color batter into each muffin cup, layering colors in order of rainbow?red, orange, yellow, green, blue and purple. Do not stir! Each cup will be about half full.

Bake 17 to 23 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes. Remove cupcakes from pans; place on cooling racks. Cool completely, about 30 minutes.

Meanwhile, divide frosting evenly among 3 medium bowls. Tint 1 red, 1 yellow and 1 blue with food colors. Refrigerate about 30 minutes.

In large (16-inch) disposable decorating bag fitted with #6 star tip, place spoonfuls of each color of frosting side by side, alternating colors and working up from tip of bag. Do not mix colors together. Starting at 12 o?clock on outer edge of each cupcake and using constant pressure on bag, pipe frosting clockwise for 3 rotations, working toward center and ending in small peak. Store loosely covered.