Transfer popped popcorn with your hands to a large bowl. Using your hands will help sift out the kernels!
In a medium saucepan over medium heat, add the corn syrup, butter, cold water, powdered sugar, and marshmallows. Stir and cook until mixture comes to a boil. Remove from heat and add food coloring, if desired.
Pour mixture over the popcorn and stir to coat. Let cool just enough so you can comfortably handle the popcorn.
Butter, grease, or wet your fingers to help keep the popcorn from sticking, then shape the popcorn into 3-inch balls. This needs to be done before it has cooled all the way.
Wrap each in cellophane or plastic wrap and store at room temperature.
Notes
If you use salted butter, just omit the pinch of salt.