Grease an 8-inch square pan with oil, line with parchment, then lightly oil the paper.
Whisk juice and gelatin in a medium saucepan. Let stand 5 minutes to bloom.
Stir in sugar, corn syrup, lemon juice, and salt. Warm over medium heat until dissolved.
Simmer gently 8–10 minutes, stirring, until thick and glossy or about 220°F.
Remove from heat. Stir in extract and food coloring, if using.
Pour into the pan and smooth the top. Cool 30 minutes, then chill until firm, 3–4 hours.
Combine coating sugar and cornstarch in a shallow bowl. Add citric acid for sour candies.
Lift candy onto a board. Cut small squares, oiling the knife as needed. Toss in coating.
Spread pieces on a rack 1 hour to dry. Store airtight at room temperature.