Preheat the oven according to the box mix directions.
Spray a 9x13 metal baking pan with cooking spray - set aside.
In a mixer bowl, combine the box mix ingredients & beat until combined.
Ladle out equal portions of the batter into 6 separate bowls.
Mix in several drops of food color - one for each bowl so you have a rainbow of batter.
Carefully drizzle batter into the prepared pan. I started with red & spread it out to cover the bottom of the pan. Then follow with orange - drizzling in a random pattern - do not stir or mix - just let them sit on top of each other. Follow with the remaining colors - yellow, green, blue, purple.
Bake according to the package instructions - remove & cool completely before adding the frosting.
Once cool, spread on the frosting, add the marshmallows & sprinkles.