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.