Pink, orange, yellow, green, blue & purple food coloring
Place the sugar, butter and half and half cream in a pot and bring to a boil over medium heat. Make sure to constantly stir the mixture. Once the mixture has come to a boil, boil for 4 minutes, constantly stirring.
Remove from the heat and add the chocolate chips and marshmallow crème. Stir until fully combined.
Remove 1/6 of the fudge at a time and dye it one color of the rainbow, starting with purple and ending with pink. Note: Dyeing the fudge one color at a time will keep the remaining, undyed fudge warm and easy to stir when you add the food coloring.
Pour the purple fudge into a 9x9 inch square baking dish lined with foil and sprayed with cooking spray. Spread the fudge all the way to the edges of the pan. Repeat with the blue fudge and continue until the fudge is layered in the order of the rainbow.
Scatter the sprinkles on top and press gently with your fingers to stick them to the fudge. Place the fudge in the freezer until fully set, about 45 minutes-1 hour.