Grease the bottom and sides of your 13 x 9 pan with butter. Set aside the pan.
In a large saucepan, melt the butter over low heat until it is just about melted. Add in the marshmallows Stir until the marshmallows are coated with the butter and completely melted. Remove pan from heat.
Add in the green food coloring and stir until the coloring in mixed in well. If you are using regular food coloring, start with 8-10 drops. I like to use Chefmaster food coloring, which gives a more vibrant color. If you use this food coloring, start with 4-5 drops. If you want a darker green, add in a couple of drops of the coloring you are using at a time and stir. Repeat until you have your desired color.
Add in the Rice Krispies cereal, two cups at a time, and stir well until evenly coated with the marshmallow mixture.
Using a buttered spatula press the cereal mixture into your pan evenly.
Grease the sides of your tree shaped cookie cutter with butter. Press down onto the Rice Krispies treats with the cookie cutter. Remove the trees from the pan while still in the cookie cutter and place them onto parchment paper. Gently press the mini M&M candy onto the trees and decorate.
Repeat cutting out Christmas trees until you have used up all of the Rice Krispies mixture. Try to cut the trees close together to use up the most cereal mixture.
Store them in a covered container for up to two days.