For the simple syrups: combine grapefruit or blood orange zest, juice, sugar, and water in a small saucepan. Heat on medium heat until sugar dissolves completely. Continue cooking on a low simmer for 3 minutes until slightly thickened. Remove from heat and let cool. Strain and refrigerate until ready to use.
For the italian sodas: add a ¼ cup of syrup to the bottom of a tall glass. Add ice to fill up the cup about half way. Add about ⅓ cup club soda, then top off with half and half.
Recipe by Wright Family Table at https://www.wrightfamilytable.com/citrus-italian-sodas-two-ways-grapefruit-blood-orange/