Line 8-inch square pan with aluminum foil, extending foil over edges. Spray foil lightly with nonstick baking spray.
In small bowl mix graham crackers, melted butter and sugar. Press crumb mixture firmly into the pan on top of the aluminum foil. Bake at 350 degrees for 6-7 minutes.
Combine white chocolate chips and sweetened condensed milk in large microwave-safe bowl. Microwave on high for 1 minute and stir. Microwave for 30 second intervals stirring between each interval until white chocolate chips are melted and mixture is smooth and creamy. Total time in microwave will be about 2-3 minutes. Add lime juice one tablespoon at a time, lime zest and green food coloring (if desired) stirring continuously. Try to work quickly.
Pour into prepared pan over the graham cracker crust. Cover and refrigerate 1-1 1/2 hours or until firm. Carefully remove from pan and peel off the foil. Using sharp thin knife cut into squares.
Heating the knife in hot water really helps in cutting the fudge. Make sure you dry the knife well before cutting the fudge.