Pop popcorn and remove any unpopped kernels, then place in a very large bowl.
Melt butter in a large saucepan over low heat, add marshmallows, and stir until fully melted and smooth.
Stir in food coloring and vanilla extract.
Pour marshmallow mixture over popcorn and stir quickly to coat.
Scatter M&Ms into the popcorn mixture and stir to distribute.
Grease your hands with butter and quickly shape handfuls of the mixture into balls before they set. Work fast.
Place on parchment and let cool for 20 minutes.