a skewer; 2- to 3-inch mermaid tail silicone molds
Preheat the oven to 350 degrees F. Stand the ice cream cones upright in the cups of two 12-cup muffin tins (or bake them in batches if you only have 1 tin).
Whisk together the oil and eggs in a bowl. Add the cake mix, vanilla extract and 1 cup water and mix until no lumps remain. Divide the cake batter into 3 bowls. Tint each batter with a different color gel food coloring (pink, purple and green), adding as much as needed until the desired shade is reached.
Fill the cones with the batter, alternating colors, until halfway full (they expand as they bake). Using a skewer in an up-and-down motion, marble the batter in each cone 2 to 3 times. Bake until a toothpick inserted in the center of a cupcake comes out clean, 14 to 16 minutes. Let cool completely.
Meanwhile, microwave the pink, purple, blue and green melting candy wafers in 4 separate microwave-safe bowls (one color in each bowl) for 30 seconds, then stir. Continue heating for 5 to 10 seconds at a time, stirring, until completely melted and smooth.
Pour the melted candy into mermaid tail silicone molds (2- to 3-inch tails), alternating colors. Place in the freezer for 6 to 8 minutes to set. Continue the process until you have enough mermaid tails.
Spread the cooled cupcakes with the blue frosting. Press a mermaid tail into the center of each cupcake, about 1/4 inch into the frosting.