In a small saucepan, combine the water and sugar and heat gently, stirring just until the sugar fully dissolves without boiling, then remove from heat, add the pink grapefruit and orange peels, cover, and let steep for 10–15 minutes until aromatic, strain out the peels, and let the syrup cool completely.
In a large pitcher, add the pink grapefruit pulp and orange pulp and mash gently with a muddler just until juice is released and light citrus structure forms, keeping texture intact rather than turning it into a purée.
Add the fresh pink grapefruit juice, fresh orange juice, 120 ml of the cooled peel-infused syrup, the pinch of fine sea salt, and the ice-cold water, then stir well to combine, taste, and adjust to preference by adding more syrup if needed, keeping grapefruit firmly in the lead.
Refrigerate for 1–2 hours until fully cold and the flavors are integrated.
Fill glasses with ice, pour over the chilled pink grapefruit lemonade, and garnish with pink grapefruit slices.
