Line a 9x13 inch baking dish with parchment paper, leaving an overhang on two sides, and spray thoroughly with non-stick cooking spray. In a medium-sized bowl, sprinkle 4 envelopes of unflavored gelatin over ½ cup of cold water; let it bloom for 5-10 minutes until thick and wobbly.

In a heavy-bottomed 3-quart saucepan, combine 4 cups of granulated sugar, 1 ½ cups of light corn syrup, and the remaining ½ cup of cold water. Stir constantly over medium-high heat until the sugar is completely dissolved. Insert a candy thermometer and cook, without stirring, until the mixture registers exactly 240°F (116°C).

Immediately remove the saucepan from heat. Quickly whisk in the bloomed gelatin mixture and ½ teaspoon of citric acid powder (if using) until fully dissolved. For multiple flavors, divide the hot mixture into several heat-safe bowls; add desired food coloring and flavor extract to each and stir quickly. For a single flavor, add the chosen food coloring and 1 tablespoon of flavor extract directly to the saucepan and stir until combined.

Carefully pour the flavored and colored gumdrop mixture(s) into the prepared pan(s). If layering colors, allow each layer to set for 10-15 minutes until just firm enough to support the next. Let the pan(s) sit uncovered at room temperature for at least 8-12 hours, or preferably overnight, until the slab is completely set and firm to the touch.

Once set, use the parchment paper overhang to gently lift the entire gumdrop slab onto a large cutting board; carefully peel back the parchment. Lightly grease a sharp knife or small cookie cutters with non-stick spray. Cut the slab into 3/4 to 1-inch squares or desired shapes. Pour 1 to 2 cups of fresh granulated sugar into a shallow bowl and toss each cut gumdrop piece in the sugar until all sides are completely coated.

For optimal texture and to prevent clumping, arrange the sugared gumdrops in a single layer on a cooling rack set over a baking sheet. Let them air dry uncovered at room temperature for another 1-2 days. Once fully dried and coated, store your finished gumdrops in an airtight container at room temperature for up to 2-3 weeks. Avoid refrigeration.