Pour Rice Krispies cereal into a large bowl. Set it aside.
Add the marshmallows, butter and green food coloring to a pot. Heat it over medium-low heat and stir continuously, until the marshmallows are melted.
Pour the marshmallow mix over the cereal, and mix until everything is evenly coated.
Add the Rice Krispies mixture to a well-greased 9×13 pan. Use a piece of parchment or wax paper to press the cereal evenly into the pan. Let it cool.
Meanwhile, in a small bowl, mix the powdered sugar with a little bit of water. Add water a few drops at a time, as you don’t want the mixture to become too runny. It should be a thick paste, which will be used to decorate the Christmas trees.
Once the Rice Krispies treats have cooled, remove it from the pan. Cut the rectangle in half lengthwise, then cut it into triangles. I was able to cut 14 triangles (7 on each half).
Cut the Christmas straw to any length you’d like. I cut off about 1/3 of the straw. Once you have your desired length, dip one end of the straw into the “paste” that you made earlier. Insert that end into the bottom of a Rice Krispies treat triangle. Squeeze the cereal around the straw to make sure it’s secure.
Decorate the Christmas trees with sprinkles and holiday candy. Use the paste to attach larger candies (I found that the small sprinkles will stick to the Rice Krispies treats as is, just by pressing them down).