I'd love a recipe for key lime fudge too!! Here's the closest recipe I could find - maybe it would work as a last resort - at least it should be green (could add some food color to be sure).

Betsy

Cherry or Lime Fudge

1 pkg. (3 oz.) cherry flavored Jello
3 1/2 c. sugar
1/4 tsp. baking soda
1 1/2 c. milk
1/4 c. butter
1/2 c. chopped candied cherries
1/2 c. chopped walnuts

Combine gelatin, sugar, baking soda and milk in a 3 quart heavy saucepan. Cook and stir over medium heat until sugar is dissolved. Continue cooking over medium heat, without stirring, to a soft ball stage. Remove from heat. Add butter. Cool until mixture is lukewarm. Beat until mixture loses its gloss. Quickly stir in cherries and walnuts. Pour into 8 inch square pan. When firm cut into squares. Cooked like cherry using lime Jello instead of cherry. After it is beaten and poured into 8 inch square pan, add 4 ounce of melted semi- sweet chocolate and spread on top.