Prepare cake mix as directed on package, using whole eggs. Stir in vanilla. Divide batter evenly into 2 bowls. Stir 75 drops (about 3/4 teaspoon) food color into one bowl, tinting batter dark green. Stir 15 drops food color into second bowl, tinting batter medium green.
Spoon heaping 1 tablespoon of the dark green batter into each of 18 paper-lined muffin cups; spread gently into an even layer. Repeat with medium green batter, spreading over dark green batter.
Bake as directed on package for cupcakes. Cool cupcakes on wire rack.
For the Frosting, beat butter and vanilla in large bowl 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. Stir in 3 drops food color to tint frosting light green. Frost cooled cupcakes with Frosting.