In a large pot, melt the butter over low to medium-low heat.
Add in the marshmallows and mix until combined.
Add in a few drops of green food coloring and mix until you get the desired color. You may want to use a combination of blues, greens, and yellows to achieve the right color.
Pour in the rice cereal. Mix until combined.
Take the pot off the heat. Let cool for a few minutes as it will be easier to work with.
Take 1/3 cup of the rice krispies and with your hands form into a cone shape. Place on a piece of parchment paper and with your finger dab a few sprinkles in key areas to make it look like ornaments.
Leave to dry and repeat with remaining rice krispies until you have about 18 trees.
Once the trees are a little harder take a peanut butter cup and attach it to the cone shape with the frosting.
Let dry for awhile and then place in tupperware or cover with plastic wrap so they don't dry out.