1. Combine sugar, water, allspice and the cloves in a medium-size saucepan; bring to boiling, then simmer 5 minutes. Strain into a large pitcher; cool.
2. Stir in frozen grapefruit-juice concentrate until thawed, then add ginger ale and a few drops food coloring to tint green.
3. Pour over ice cubes in 6 tall glasses; garnish each with a slice of lemon and a maraschino cherry.