Glow in the Dark Cake

Name: Glow-in-the-Dark Cake
Ingredients: Nonstick cooking spray, for the cake pans; All-purpose flour, for the cake pans; Two 16-ounce boxes white cake mix (plus required ingredients); 5 drops electric green gel food coloring; 5 drops deep pink gel food coloring; 5 drops lemon yellow gel food coloring; 4 sticks (1 pound) unsalted butter, at room temperature; Pinch fine salt; 8 cups sifted confectioners' sugar; 1 tablespoon pure vanilla extract; 2 to 4 tablespoons milk; Two 10-ounce bags marshmallows; 4 tablespoons unsalted butter; 1 1/4 cups tonic water (with quinine), at room temperature; 3 drops electric green gel food coloring; 3 drops deep pink gel food coloring