In a medium saucepan, whisk together sugar and zest. Whisk in cornstarch and salt. Add juice and ¼ cup water, whisking until smooth. Heat over medium heat until sugar is completely dissolved. Whisk in egg yolks and gelatin. Increase heat to medium-high; cook, whisking constantly, until mixture comes to a boil and thickens, about 5 minutes. Remove from heat and stir in butter and food coloring.
Strain mixture through fine mesh sieve into a bowl. Press plastic wrap directly onto surface of curd. Let cool to room temperature and refrigerate until chilled.
Notes
*We used Watkins Food Coloring, which contains no artificial dyes.
Recipe by Victoria at https://victoriamag.com/champagne-grapefruit-layer-cake-recipe/