1. Mix the frosting with a few drops of food coloring in a small bowl until you have a rich, even color. Add more food coloring until you get your desired color.
2. Smear some frosting around the rim of each milkshake glass, making a 1-inch border. Use a paper towel to clean up the rim.
3. Scatter sprinkles over the frosting on the rim of each glass. Transfer the glasses to the freezer while making the milkshakes.
4. Process the ice cream and milk together in a blender until smooth. Divide the milkshake between two glasses.
5. Top each milkshake with whipped cream, sprinkles and the halloween candy of your choice. Serve with a wacky straw for sipping.