Line a 9x9 pan with parchment & then spray it with cooking spray or oil- set aside.
In a medium bowl - combine the semi-sweet chocolate chips & the food coloring - set aside.
Combine the white chocolate chips and vanilla in the mixer bowl & set aside.
In a large stockpot, combine the sugar, marshmallow creme, butter, and evaporated milk. Bring to a rolling boil over medium heat.
Continue to boil about 4-5 full minutes - bringing to 234 degrees on a candy thermometer.
Remover from heat & quickly & carefully pour this hot mixture over white chocolate chips in the mixer bowl.
Beat on medium-high until silky smooth - but not too long that the fudge cools too much (you need to keep it relatively hot for the remaining steps).
Remove ½ of the fudge mixture & spread it out into the prepared parchment lined pan.
Add the semi-sweet chocolate & food coloring to the remaining white chocolate fudge in the bowl & beat until all chocolate is melted & combined.
Drop spoonfuls of the mixture over the top of the white fudge already spread in the pan.
Take a butter knife & drag through to help combine the colors & give it a marbleized look.
Smooth out the top with a spatula & place in refrigerator to set a minimum of 2 hours but preferably overnight before cutting.
Store in an airtight container at room temperature.