Grease the top surface of enough cupcake tins to hold 18 cupcakes, and line the tins with white paper cupcake liners. Set the tins on baking sheets.
Prepare the cake mix according to package directions and divide it evenly among 18 liners. Filling the cups more than usual will give each of the cupcakes a mushroom cap. (And greasing the tops of the tins will make the larger crown easier to remove.) Bake according to package directions and let cool completely.
Spoon the frosting into a mixing bowl and add enough red paste coloring (as much as 1 teaspoon) to achieve a bright-red shade.
Ice the cupcakes liberally with red frosting. Decorate the frosted cupcakes with assorted round white candies. Makes 18 cupcakes.