Melt butter and corn syrup together in a heavy pan. Add sugar and Jello, bring to a boil over medium-high heat. Let boil for 5 minutes, remove from heat.
Carefully pour Jello mixture over popcorn – mixture is HOT NASTY LAVA, so it’ll burn you if you’re not careful. Mix well.
Spread popcorn out on a greased cookie sheet, and allow to cool. (Some people like to bake this for 10 minutes at 350F… personally, I like the slight gooeyness it keeps without being baked!).
Make a second batch in another flavor/color, and also allow to cool/dry. Break popcorn up into pieces, and either serve side by side in a bowl, or mix together.
If you’re not using the popcorn right away, store it in an airtight container, or package into baggies.