Prepare the grapes.
Wash the grapes under cool running water, then dry thoroughly with clean towels. Let them air-dry until no moisture remains.
Insert a toothpick securely into each grape. Line a baking sheet with a silicone mat or lightly greased parchment paper.
Keep the grapes dry and at cool room temperature before dipping. Freezing can cause condensation, which interferes with the coating.
Cook the candy syrup.
In a heavy-bottomed saucepan, combine the sugar, water, corn syrup, and lemon juice.
Heat over medium heat, stirring gently until the sugar dissolves. Once boiling, stop stirring.
Using a candy thermometer, cook to 300°F, the hard-crack stage. Keep the thermometer tip in the syrup without touching the bottom of the pan.
Add flavor and color.
Remove from the heat and let the bubbling settle briefly.
Carefully stir in the candy flavoring according to its package directions. Add a little white gel coloring if you want an opaque base.
Add a small amount of your chosen color and gently swirl for a marbled effect, or stir fully for an even color.
Dip the grapes.
Hold each grape by its toothpick, keeping your fingers clear of the hot syrup. Dip and rotate to coat.
Lift, let excess syrup drip back into the pan, then place on the prepared baking sheet.
Decorate immediately.
If using sprinkles or crushed candy, add them before the coating hardens. Work with one grape at a time.
Cool and serve.
Allow the coating to harden and cool completely before eating. Remove toothpicks before serving.
Enjoy soon after making for the best crunch. If preparing ahead, refrigerate in a single layer in an airtight container and enjoy within 24 hours; moisture may make the coating sticky.