Glistening cubes of gelatin flavored with grape juice make a cool, light, and refreshing dessert.
Combine grape juices. Pour 1 cup combined juice into another bowl; sprinkle gelatin over. Let stand to soften, 5 minutes.
Bring sugar and 1 cup juice to a boil over medium-high heat, stirring to dissolve. Add to gelatin mixture; whisk to dissolve gelatin.
Stir remaining 2 cups juice into gelatin mixture. Pour into a 9-by-13-inch Pyrex pan, cover, and refrigerate to set, 2 hours.
When ready to serve, cut gelatin into 1-inch cubes, and serve in tall glasses.