In heavy saucepan on medium heat melt butter then add sugar, corn syrup, cream of tartar and salt. Increase temperature to medium-high and bring mixture to a boil, stirring constantly to dissolve sugar. Once mixture boils, add in food coloring and stop stirring. Boil for 5 minutes (Do not stir). The mixture should be about 250-260 degrees. Remove from heat and carefully stir in vanilla and baking soda. Working quickly and while the mixture is foamy, pour mixture over popped corn and gently stir to coat popcorn. Place popcorn into a large roaster or rimmed baking sheet (lined with parchment paper or sprayed with cooking spray) and bake at 200° F for 1 hour, stirring every 15 minutes. Check often to make sure it doesn't burn. It should not change colors. It is okay to pull it out early if needed.