Melt butter and marshmallows in a heavy bottom pan.
Mix in rice cereal and green food coloring to desired green color. Press down evenly with a greased spatula onto a greased parchment lined rimmed baking pan to cool. At least an hour or so.
Cut Rice Krispie treats into triangles and insert an pretzel stick for the tree stem.
Drizzle white icing (store bought is fine or you can make your own with powdered sugar, and a bit of milk or cream) and decorate with candies.