Cook and stir (low heat) until sugar dissolves. Bring to a boil and cook for 4 minutes and stir. Remove from heat and stir in chips and marshmallow until smooth. Remove 1 cupful. Add extract and food color to rest of mixture, stir until blended. Pour in greased pan. Drop reserved cupful by tablespoons over top. Gently cut through with knife and swirl. Cover and refrigerate.
Note: for thin fudge use a 13 x 9 inch pan, for thicker fudge use a smaller pan.