Line an 8x8 pan with foil so that it is sticking out over the edges.
In two separate sauce pans, both over medium low heat, melt the white chips and the chocolate chips.
Add half of the sweetened condensed milk to one and half to the other.
Stir both pans until they are fully melted.
Add the mint extract and the green food coloring to the white chips and stir until fully combined.
Remove both pans from the heat and add a few spoonfuls at a time of each one to the pan. Continue adding randomly until both sauce pans are empty. Shake the 8x8 pan a little to make it even. Very gently run a butter knife a few times through to give it a swirled effect.
Refrigerate overnight. Pull the foil (with the fudge) out of the pan and away from the fudge. Cut into square pieces. Serve or store in an airtight container.