Instructions

for the crust

Adjust oven rack to middle position and heat oven to 350 degrees. Whisk flour, sugar, and salt together in large bowl. Add melted butter and stir with spatula until dough forms. Using your hands, crumble two-thirds of dough over bottom of 9-inch tart pan with removable bottom. Press dough to even thickness in bottom of pan. Crumble remaining dough and scatter evenly around edge of pan, then press crumbled dough into fluted sides of pan, pressing so dough rises about ⅛ inch above sides and making sure dough is pressed into corner around base of pan. Place pan on rimmed baking sheet and bake until tart shell is golden brown, 25 to 30 minutes, rotating pan halfway through baking. Let tart shell cool on sheet while preparing filling.

for the filling

Whisk sugar, eggs and yolks, cornstarch, and salt in medium bowl until smooth. Bring grapefruit juice and lemon juice to simmer in small saucepan over medium-high heat. Whisking constantly, slowly pour hot juice mixture into egg mixture and whisk until smooth; transfer mixture to now-empty saucepan.
Cook egg mixture over medium heat, stirring constantly with rubber spatula, until mixture clings to spatula and registers 165 to 170 degrees in multiple places, 3 to 5 minutes. Off heat, stir in butter and food coloring until butter is melted and fully incorporated. Strain curd through fine-mesh strainer into medium bowl, gently pressing on solids to extract curd. Whisk in grapefruit zest.
Pour filling into tart shell (shell needn't be fully cooled). Bake tart on sheet until edges are set and center 5 inches jiggle slightly when nudged, 16 to 19 minutes. Let tart cool completely on wire rack, about 1 hour. Refrigerate, uncovered, until filling is well chilled and firm, about 2 hours. (Tart can be refrigerated for up to 3 days; after tart is well chilled, cover with inverted bowl. Blot away any accumulated moisture with paper towel before decorating and serving.)
When ready to decorate, remove outer metal ring of tart pan, slide thin metal spatula between tart and pan bottom to release, then slip tart onto platter.

for the topping

Using stand mixer fitted with whisk attachment, whip cream and sugar on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until stiff peaks form, 1 to 3 minutes. Transfer whipped cream to pastry bag fitted with pastry tip. Pipe decorative border around perimeter of tart as desired. Transfer any remaining whipped cream to small serving bowl. Cut tart into wedges and serve, passing extra whipped cream separately.