Grease your baking dish liberally with butter.
Melt the butter over low heat in a sauce pan.
4 Tbsp Butter
Add the marshmallows to the butter and stir constantly until they melt into goo.
5½ Cups Marshmallows
Add the cereal and stir until it's all well-mixed. Add this to the baking dish and press it flat to form your bars, and set these aside to set for at least 20 minutes.
6 Cups Rice Krispies
Divide your candy melts into microwave-safe bowls, one for each color you want to use for your decorating.
10 Oz. White Chocolate Candy Melts
Microwave the bowl you want to use for about 30 seconds to start melting the wafers. Microwave for 10 seconds at a time after until they're fully melted.
Stir in food coloring to make the color you want. How much you use depends a lot on how much chocolate you're using and what kind of food coloring you're using.
Food Coloring
Cut your Rice Krispie Treats into bars and dip them in your chosen color. While the chocolate is still warm and melty, use a small spatula to spread chocolate over the front and sides (not the bottom, otherwise it will stick to whatever you set it down on later!) and decorate with sprinkles so they stick when the candy solidifies.
Sprinkles, Candy Eyeballs
Repeat the last two steps for whatever colors and decorations you want to use.