The ingredients will need to be divided by six, one for each color. For each layer, place 1 tablespoon of butter and 2 1/2 ounces of marshmallows in a large bowl sprayed with cooking spray. Microwave for 1 minute (or heat in a saucepan on the stove-top over medium-low heat).
Remove from the heat and stir very well until the butter and marshmallow mixture is smooth. Add a few drops of your food coloring and stir well. Add more drops until desired shade is achieved.
Stir in 1 1/4 cup of Rice Krispie treats using a spatula sprayed with cooking spray. Mix until the cereal is fully coated by the marshmallow mixture.
Spread the colored mixture evenly over the bottom of the prepared baking pan. An off-set spatula sprayed with cooking spray makes this very easy to do.
Repeat with the other colors, cleaning the bowl and spatulas each time. Here, I did purple first, then blue, green, yellow, orange, and red.
Let cool, at least 2 hours. Remove from the pan by grasping the edges of the foil and cut into squares.
The marshmallow amount by weight is an estimate. If you purchase a 16 ounce bag, divide the marshmallows by six before you begin. That way, if 2 1/2 ounces is less than 1 /4 cup mini or 12 regular, you won't run out of marshmallows before your final layer. Or, buy an extra bag just in case, as the few extra marshmallows used won't make a difference.
I suggest Americolor soft gel paste food coloring.
Note the layers will fill the entire baking pan, so be sure and line with foil that hangs over the edges so you can easily remove the treats.