Line an 8x8 inch square baking dish with parchment paper, leaving some overhang for easy removal later.
In a medium saucepan over low heat, combine the sweetened condensed milk and semi-sweet chocolate chips. Stir continuously until the chocolate is melted and the mixture is smooth.
Once melted, remove from heat and stir in the vanilla extract and sea salt. Pour half of the mixture into the prepared baking dish and spread it evenly.
In a separate bowl, melt the white chocolate chips using the same method (low heat, stirring constantly). Once melted, you can add a few drops of green food coloring if desired for an extra pop of color.
Pour the white chocolate mixture over the semi-sweet fudge already in the dish, spreading it evenly.
In the remaining half of the semi-sweet fudge, gently fold in the finely chopped pistachios. Carefully dollop spoonfuls of this mixture on top of the white chocolate layer. Use a knife or spatula to gently swirl the layers together, creating a marbled effect.
Cover the dish with plastic wrap and refrigerate for at least 4 hours, or until the fudge is firm.
Once set, lift the fudge out of the dish using the parchment overhang. Cut into squares and serve.
Notes
Refrigerate for at least 4 hours to ensure the fudge sets properly.