Pour the Cheerios into a large mixing bowl and set aside.
In a large saucepan, melt the butter and marshmallows on low-medium heat, constantly stirring until the mix is smooth.
Add the melted marshmallow mixture and the green food coloring to the Cheerios. Stir until it's evenly coated.
Generously coat your hands with cooking spray and grab a handful of the Cheerios mixture. Shape into a tree. Place them on a sheet of waxed paper. While they are still sticky, place red and green M&M candies on the trees as ornaments and a yellow M&M on the top of each one for the star.
Refrigerate about an hour before serving.