Preheat oven to 350 degrees F.
Prepare mini bundt cake pan by coating with cooking spray.
Prepare cake mix according to box and the divide into 6 small bowls.
For each bowl of cake batter, mix in one of the food colors/gel.
Put one tablespoon of red batter in the mini bundt cake molds.
Next top with one tablespoon of orange batter, then one tablespoon of yellow, green, blue, and purple.
Bake for 20 minutes or until toothpick comes out clean.
For marshmallow buttercream, cream together butter and vanilla.
Add the powdered sugar and mix until frosting consistency.
Add marshmallow cream and mix about 2 minutes.
Serve buttercream alongside cakes.