Heat oven to 180°C (170°C for dark or nonstick pans). Grease and flour bottom and side of 23cm round cake pan. Place paper baking cup in each of 12 regular-size muffin cups.
Make cake batter as directed on box. Spread half of batter in round pan; spoon remaining batter into muffin cups. Bake as directed on box. Cool completely, about 1 hour.
In small bowl, place 1 tablespoon frosting; stir in 2 drops yellow food color. In another small bowl, place 60 grams frosting, 3 drops yellow food color and 1 drop red food color; stir to make orange frosting.In third small bowl, place 110 grams frosting; stir in 7 drops blue food color. To remaining frosting in container, stir in 7 drops green food color.
Trim off rounded top of cake layer; place on serving plate, cut side down. Frost top and side with green frosting. Frost 7 cupcakes with blue frosting, 4 cupcakes with orange frosting and 1 cupcake with yellow frosting. Place 1 blue cupcake on center of green frosted cake. Place remaining blue cupcakes, sides touching, in circle around center cupcake. Place 2 rows of 2 orange cupcakes on top center of blue cupcakes. Place yellow cupcake on top center. Press candies into frosting to decorate. Store loosely covered at room temperature.