Preheat oven to 200 degrees. Spray a 9x13 baking dish with non-stick spray or coat with butter.
Pop enough popcorn to fill a 9x13 baking dish. Pop a couple extra cups so you can add to the mix if needed. It's important to keep the popcorn warm in the oven while completing the following steps.
Melt the butter and marshmallows together in a large pot.
Once the marshmallows are fully melted, turn off the heat and add in the vanilla and food coloring. I started with a couple drops of red and then added more until I got the color I wanted.
Pull the baking dish full of popcorn out of the oven. Drizzle the marshmallow and butter mixture over the popcorn.
Use your hands or a wooden spoon to mix up the popcorn and marshmallow mixture so that the popcorn is evenly coated. If you are using your hands (I do), spray your hands with the baking spray or cover them in butter - this is to protect your hands from the heat AND to keep the marshmallow mixture from sticking to you. After the popcorn and marshmallow mixture are thoroughly combined and cooled slightly, add in the M&Ms. You need to wait or the candy will melt and turn your popcorn balls brown from the chocolate.
Recoat your hands and grab a handful of popcorn and start forming it into a ball. You will need to squeeze firmly while making the balls. Make them any size you want to but make sure they are pressed together well or they will fall apart.
Wrap the balls in plastic wrap or bag them up - they're yummy no matter what you do! Enjoy!