Perhaps Pink
1 oz Rhubarb Liqueur
1/4 oz Simple Syrup (cardamon is good)
1/4 oz lemon juice
Shake over ice. Strain and top with grapefruit soda, we like to use the Pelligrino Pomelo. Garnish with candied rhubarb slice if you feel like fussing, or a curl of lemon peel.