Line an 8x8 inch baking pan with parchment paper and set aside for now.
Set three separate medium bowls with three separate stirring spoons on the countertop.
Place the white chocolate chips, sweetened condensed milk, and salted butter into a large saucepan.
Heat on medium, stirring often, until the chocolate is completely melted and the mixture is glossy, about 4 minutes. Do not overcook the mixture. As soon as the chocolate chips are melted you should be good to go. The mixture should be thick.
Remove from the heat and pour ⅓ of the fudge mixture into each of the three separate bowls.
Quickly stir in the desired amount of pink food coloring to one bowl, purple coloring into the second, and blue food coloring to the third.
Immediately use a ¼ cup measuring cup to scoop the fudge into the baking pan, alternating between colors.
Create swirls throughout the fudge with a knife. Be careful not to mix the colors too much, otherwise the contrast of the colors won’t be as noticeable.
Quickly sprinkle with marshmallow bits and rainbow sprinkles. You may have to press them gently into the fudge depending on how much it has cooled down.
Refrigerate for 2-3 hours or until the fudge sets completely.
Once set, remove fudge from the pan using the parchment paper, cut into squares, and serve.
To Store: To store your fudge, cover it and keep it at room temperature for up to one month.
To Freeze: You can individually wrap and freeze pieces of this fudge for 3-4 months. Thaw at room temperature before serving.
To make sure you can get your fudge out of the pan, I find it easiest to use parchment paper sprayed with cooking spray. Do not, however line your pan with foil, as it will stick to the fudge.
Use 3 heatproof bowls and heat them up in the microwave prior to pouring the white chocolate mixture into them. If the bowls are warm, the white chocolate mixture won’t cool down as quickly. Heat the bowls until they feel hot, but still cool enough to handle.