Spray a 9 x 13-inch baking dish with nonstick cooking spray. Set aside.
In a large pot, melt butter over medium heat. Add sweetened condensed milk and stir to combine. Bring the mixture to a simmer, stirring constantly
Add bag of plain and approximately half the bag of the colored marshmallows and stir until melted.
Remove the pot from heat and add the rice cereal with a clean rubber spatula.
Sprinkle funfetti over the mixture reserving about 2 Tablespoons
add reserved colored marshmallows
Stir until well-coated.
Transfer the mixture to the prepared baking dish and press into an even layer.
Add final funfetti onto the top and lightly press into the mixture to adhere
Cool to room temperature before cutting and serving.