1. Cut the grapefruit into two equal halves.
2. Using a grapefruit knife, loosen the segments in each half.
3. Into a basin, squeeze the flesh and juice.
4. Stir in the castor sugar and creme de menthe until the green color blends evenly and the sugar dissolves.
5. Cover and refrigerate to chill.
6. Into six individual glasses, spoon the mixture.
7. Serve chill.