Bring 2 cups water to a boil in a saucepan. Add cherry gelatin and 1 packet unflavored gelatin; stir until dissolved. Remove from stove. For pink: Transfer 2/3 cup red gelatin mixture to a bowl and whisk in yogurt. For white: Pour grape juice into a bowl. Sprinkle on remaining unflavored gelatin and let stand 5 minutes. Microwave on high about 1 minute, until gelatin has dissolved.
To assemble, pour 2/3 cup of red mixture into an 8-inch square baking pan. Refrigerate until set, about 15 minutes. Top with half of pink and white mixtures, refrigerating for 15 minutes after each addition. Repeat process with remaining gelatin, ending with red. Refrigerate until completely set, about 2 hours or overnight.
Dip bottom of the pan in hot water to loosen. Invert onto a cutting board and slice into 1-1/2-inch cubes.