Spray a 9x13-inch baking pan with cooking spray.

Melt butter in a large saucepan over low heat. Add marshmallows and stir continually until completely melted and smooth, 3 to 5 minutes. Add food coloring, 3 to 4 drops at a time, until you reach the desired color. Stir until color is uniform. Remove from heat and add crispy rice cereal. Stir with a buttered spatula until cereal is completely coated and evenly green.

Pour cereal mixture into the prepared baking pan and press in firmly. Add sprinkles to the top and press in. Allow to cool and set, 20 to 30 minutes.

Cut cereal mixture in the pan in half lengthwise to create 2 sections. Cut each section into approximately 6 or 7 small triangle tree shapes and transfer them to a plate. Carefully twist 1 pretzel stick into the base of each triangle as a "trunk;" if the triangle shape separates, squeeze it back together.

Pipe frosting onto triangles as garland and top each with a candy star.