Thaw the bananas: Remove from freezer and thaw completely, 1-2 hours.
Mash the bananas: Once fully thawed, thoroughly mash until they form a thick pulp.
Sauté aromatics: Heat 2 tablespoons annatto oil in a medium saucepan over medium heat. Add garlic and onion and sauté until translucent, about 2 minutes. Add tomato paste and fry for 2 minutes.
Combine ingredients: Add mashed bananas and vinegar, combining well. Add water, corn syrup, soy sauce, paprika, salt, pepper, and red pepper flakes (if using).
Simmer: Cook on low heat until the vinegary scent fades, 6-8 minutes. The ketchup should coat a spoon similar to store-bought ketchup.
Finish and cool: Add sugar and food coloring (if using) and mix well. Turn off heat and allow to cool completely.
Blend: Use an immersion blender or food processor to blend until smooth.
Store: Transfer to clean mason jars and refrigerate for 2-3 days before using to allow flavors to develop.