Line an 8-by-8-inch baking dish with foil, leaving an overhang on all four sides. Divide the 2 containers of vanilla frosting equally between 4 microwave-safe bowls. Microwave each bowl of frosting until liquid, 20 to 30 seconds. Put each color of melting candy in a separate microwave-safe bowl. Microwave each color in 30-second intervals, stirring in between each, until melted and smooth. Pour each color of melted candy into a bowl of frosting and mix well. Quickly spoon each color into the prepared baking pan, swirling the colors together as you work. Tap the pan on a work surface to level the candy, then cover the top with colorful sprinkles. Gently press the sprinkles down into the fudge with your hands. Refrigerate until set, at least 1 hour. Lift the candy from the pan using the overhanging foil edges, remove the foil and cut into squares. If you like, package the fudge in small windowed boxes and tie with ribbon.