For the cupcakes, preheat oven to 350°F. Prepare cake mix as directed on package, adding vanilla. Remove about 1 3/4 cups of the batter to small bowl. Stir in black food color. Set aside. Add green food color to remaining batter. Spoon green batter into 16 paper-lined muffin cups. Spoon black batter into 8 paper-lined muffin cups.
Bake 20 to 22 minutes or until toothpick inserted into center comes out clean. Cool completely on wire rack.
For the frosting, beat butter and vanilla in large bowl with electric mixer on medium speed until light and fluffy. Gradually add confectioners' sugar, beating well after each addition and scraping sides and bottom of bowl frequently. Add milk; beat until light and fluffy. If frosting is too thick to spread, gradually add additional milk.
Remove 1/4 cup of the frosting to small bowl. Tint frosting gold using 10 drops yellow and 1 drop each red and green food color. Divide remaining frosting between 2 bowls. Tint one bowl to desired shade of green using green food color. Leave remaining bowl of frosting white.
To assemble cupcakes, remove center of black cupcakes using 1- to 1 1/2-inch round cookie cutter. Cut centers in half crosswise to make 2 black “pots”. Spread small amount of green frosting on bottom of each pot. Pressing lightly to stick, place one pot on top of each green cupcake. Spread small amount of gold frosting on top of pots. Add gold sprinkles. Use a small sharp knife to make a slit in the center of each gold pot, large enough to insert one end of rainbow candy strip. Make another matching slit on top of green cupcake. Insert ends of rainbow candy in slits. Pipe white frosting around base of rainbow to resemble clouds. Pipe green frosting around pot of gold using small star tip, or spread frosting and use a fork to gently touch frosting in different directions to resemble grass spikes.