Line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides.
In a medium saucepan, combine sugar, heavy cream, and butter. Cook over medium heat, stirring constantly until the butter is melted and the sugar is dissolved.
Bring to a boil, and cook, stirring constantly, for 5 minutes. Remove from heat.
Stir in marshmallow creme and white chocolate chips until melted and smooth.
Stir in peppermint extract and green food coloring (if using).
Fold in mint chocolate chips.
Pour the fudge into the prepared pan and spread evenly.
Refrigerate for at least 2 hours, or until firm.
Cut into squares and serve.
Notes
Store fudge in an airtight container at room temperature for up to a week, or in the refrigerator for up to two weeks.