Preheat the oven to 250 degrees F (120 degrees C). Spread popcorn into 1 large or 2 smaller greased roasting pans.
Combine brown sugar, butter, corn syrup, and food coloring in a medium saucepan over medium heat. Stir constantly until boiling. Let boil undisturbed for 5 minutes. Remove from heat. Stir in vanilla extract and baking soda; mixture will foam up then subside.
Pour mixture over popcorn and stir until coated.
Bake in the preheated oven, stirring every 15 minutes, for 45 minutes. Stir popcorn as it cools to break it apart.
