Prepare cake mix as directed on package, using whole eggs. Stir in vanilla. Divide batter evenly into 3 bowls. Stir 50 drops (about 1/2 teaspoon/2 ml) food colour into one of the bowls, tinting batter dark green. Stir 15 drops food colour into another of the bowls, tinting batter medium green. Stir 2 drops food colour into last of the bowls, tinting batter light green.