Prepare peppers: Wear gloves when handling jalapeños. Remove stems from all peppers. For milder jelly, remove seeds and white membranes from all jalapeños. For more heat, leave seeds in 3-4 peppers.
Chop peppers: Process jalapeños and bell peppers in food processor until finely diced but not pureed. Measure exactly 3 cups chopped peppers, draining excess liquid.
Cook mixture: Combine chopped peppers, vinegar, and salt in large heavy-bottomed pot. Bring to rolling boil over high heat, stirring frequently.
Add sugar: Once boiling hard, add all sugar at once. Return to full rolling boil that cannot be stirred down. Boil exactly 10 minutes, stirring constantly.
Add pectin: Remove from heat and stir in liquid pectin immediately. Return to high heat and bring back to rolling boil. Boil hard for exactly 1 minute while stirring constantly.
Finish: Remove from heat and skim off foam. Add food coloring if using, stirring gently to distribute.
Can or store: For canning, ladle hot jelly into hot half-pint jars leaving ¼ inch headspace. Wipe rims, apply lids and bands. Process in boiling water bath for 10 minutes. For refrigerator storage, pour into clean jars and cool before refrigerating.