Soak gelatine in cold water. Squeeze yellow grapefruit (should yield about 200 ml juice). Peel the pink grapefruit so that the white skin is completely removed. Cut the grapefruit into slices or fillets as desired. Drain on kitchen paper. Mix yoghurt, grapefruit juice, sugar and vanillin sugar. Dissolve gelatine over low heat while stirring and allow to cool slightly.
Stir in the yoghurt mixture bit by bit. Whip cream until stiff and fold into the yoghurt cream. Leave to stand for a while. Layer cream and grapefruit flesh in glasses. Place in the refrigerator for about 1 hour. Decorate with grapefruit wedges and lemon balm leaves just before serving