Preheat oven to 350 degrees. Line the cupcake pans with the paper cupcake liners.
Prepare and bake the cake mix according to the package instructions. Remove from the oven, transfer to a wire rack, and let cool completely.
Arrange 2 rows of cupcakes into an arched rainbow shape on the cardboard or tray. Spoon the can of vanilla frosting into a resealable plastic bag. Snip a small corner from the bag and twist the top. Pipe frosting on top of each cupcake. Spread the frosting -- smoothing it between the cupcakes -- to look like one big cake.
Arrange similar-color M&M'S® Brand Milk Chocolate Candies for Easter into five curved lines stretching from one end of the rainbow to the other.
Cut several marshmallows in half. Arrange the halves in groups of three around the bottom and top edges of the rainbow for clouds!