Place popcorn in a large roasting pan; keep warm in a 250° oven. Meanwhile, in a large heavy saucepan, combine the sugars, water, corn syrup, vinegar and salt. Cook and stir over medium heat until mixture comes to a boil. Cook, stirring occasionally, until a candy thermometer reads 260° (hard-ball stage).
Remove from the heat; stir in butter until melted. Stir in food coloring. Drizzle over warm popcorn and toss to coat. Cool. Break into pieces. Store in an airtight container.
We recommend that you test your candy thermometer before each use by bringing water to a boil; the thermometer should read 212°. Adjust your recipe temperature up or down based on your test.