Prepare cupcakes according to the box’s directions - allow the cupcakes to cool completely before decorating.
Add butter, icing sugar and milk to a large bowl. Use an electric mixer to beat together all ingredients, starting on low and gradually increasing to high.
Beat ingredients until smooth, creating semi-stiff peaks.
Divide buttercream into 4 separate bowls. Add one color of gel food coloring to each bowl. To achieve a rich color, add about 2-3 small drops (a bit less than ¼ of a teaspoon).
Mix each bowl of buttercream until completely tinted.
Attach Wilton icing tip #21 to a piping bag. Scoop colored buttercream into the piping bag, alternating colors.
Frost each cupcake.
Decorate each cupcake with cereal, using mainly the marshmallow shapes. Enjoy!
Notes
**We recommend splitting the portions in half, as most cake mixes yield about 24 cupcakes. This way, you can make one dozen.