Place 2 tablespoons of butter and 1 ½ cups mini marshmallows in a medium mixing bowl.
Microwave on high for 1.5-2 minutes, until butter is melted and marshmallows are soft.
Add your first food color (I used the traditional rainbow order: red, orange, yellow, green, blue, purple) and whisk together quickly.
Fold in 2 cups rice cereal.
Spread mixture into the bottom of your prepared baking dish, using a greased spatula to spread. I also used a piece of wax paper to help spread the layers flat without a sticky mess.
Repeat steps 2-5 for the remaining colors of the rainbow.
When all the layers are prepared, allow to cool. Then cut into squares for serving