In a large bowl, combine popcorn and pretzels. Set aside. In a medium saucepan, bring sugar, 1/2 teaspoon salt, and water to a boil over medium high heat. Boil, undisturbed, until mixture is amber in color, for about 8-12 minutes. Remove pan from heat and slowly pour in the heavy cream. Be careful because the mixture will bubble up. Stir until smooth and add vanilla extract, marshmallows, and drop of red food coloring-if using. Stir until marshmallows are melted and mixture is smooth.