Preheat oven to 200°F. Place popcorn in large bowl.
Bring sugar, butter and corn syrup to boil in medium saucepan on medium heat. Cook and stir 4 minutes. Stir in food colors and baking soda. (Mixture will foam up.) Remove from heat. Pour over popcorn in bowl. Toss to coat evenly. Spread popcorn evenly on foil-lined 15x10x1-inch baking pan.
Bake 1 hour, stirring every 15 minutes. Cool completely on wire rack. Store in airtight container.