Tint five separate bowls of vanilla frosting with food coloring as follows: 1 cup red and 1/2 cup each orange, yellow, green, and purple. Reserve the remaining frosting for the clouds. Top the mini cupcakes with frosting as follows: 15 red, 12 orange, 9 yellow, 7 green, and 5 purple. Generously frost the large cupcakes with the untinted frosting and cover each with mini marshmallows. Arrange the cupcakes on a platter in a half rainbow, placing five of the cloud cupcakes at one end, or a full arc with three clouds at each end.