Line a cookie sheet with parchment or waxed paper; set aside. In a medium microwave-safe bowl, combine semisweet chocolate pieces, 1/3 cup sweetened condensed milk, and 1 tablespoon water.
Microwave, uncovered, on 100% power (high) for 1 minute; stir. Microwave about 1 minute more, or until chocolate is melted and mixture is smooth, stirring every 30 seconds. Repeat the process in a separate bowl with the white chocolate pieces and the remaining milk and water.
Pour mixtures onto prepared cookie sheet and spread into a 9x6-inch rectangle. Add in peppermint candy. Create the marbled look by dragging a fork through the prepared mixture on the cookie sheet.
Chill fudge about 30 minutes or until firm. Cut fudge into 1-1/2-inch squares. Makes 24 pieces