You need 2-4 cups of plain popcorn (no butter or seasonings) for each color (depending on how much color you want). We used about 4 cups per color. You can also add nuts (or other add-ins after the baking is complete). You also need Jello, Caro syrup, granulated or brown sugar and butter.
Grease or spray non-stick spray into the bottom of a shallow dish, preferably a glass dish. You can also line a pan with parchment paper, which will definitely be my preference next time. Spread the popcorn in the pan and set aside.
Place 2 tablespoons of butter in a small saucepan and add 1 1/2 tablespoons of corn syrup. Heat over medium heat, stirring constantly. Add 1/4 cup granulated or brown sugar and half of a 3 oz package of Jello, your choice of flavor (color). Stir until caramelized, being careful not to burn. Remove from heat and immediately drizzle over popcorn. (Don't be slow, this stuff clumps very fast.) You can toss it a little to coat as much as you can.
Bake in the oven at 250 degrees for 12-15 minutes, stirring a couple of times during baking. Pour onto wax paper, spread out and let it cool.
The Jello flavors make it really fruity. I liked combining the flavors - it was like Skittles. You can store it in Ziploc bags and it will last about a week.
I think the first time you try a recipe it's a good idea to keep some notes about what you would do differently:
1. I would use less popcorn for each color so you get more coated.
2. Substitute brown sugar for granulated sugar
3. Use parchment paper to line the pan for baking. You can then move the parchment paper around to stir the popcorn during baking.
4. Important clean up tip - pour boiling water to rinse/clean pans and spoons. You will never be able to scrub this stuff from the bottom of your pans, but pouring boiling water into the pans will work very nicely.