Put the popped popcorn in a large mixing bowl, removing any unpopped kernels.
Melt the butter in a medium-size saucepan over medium-low heat. Add the marshmallows and stir continually until they melt and turn into a smooth sauce. Stir in the cherry gelatin powder. Use a plastic spoon for this step because the pink gelatin can stain a wooden spoon.
Immediately pour the pink marshmallow over the popcorn, and gently stir to evenly cover the popcorn.
Let the popcorn cool for a few minutes before serving.