Butter (or line with parchment paper) a 9 x 13 or similar casserole dish.
In a large pot, melt butter over medium-low heat until just melted.
Turn the heat off and stir in the marshmallows. The residual heat from the melted butter should be enough to melt them, but if it is not, turn it back on low until the marshmallows are smooth.
Add in 20 drops of green food color and mix well.
Add in the cereal, and stir until well blended and completely coated.
Pour the mixture into a pan and spread out.
Add Trix cereal pieces throughout to act as ornaments on the trees.
Allow to cool for 20 - 30 minutes in the fridge before cutting.
Cut in half lengthwise, then cut into triangles. You will have two extra pieces on each side.