Prepare the crust: Preheat oven to 275°F. Place crushed crackers, powdered milk, sugar, and salt in a large bowl; toss to combine. Stir in melted butter. Press mixture on bottom, up sides, and over rim of a 9-inch pie plate. Bake in preheated oven until dried and lightly toasted, about 20 minutes. Cool on a wire rack until ready to use.
Prepare the grapefruit curd: Combine sugar, cornstarch, and salt in a medium saucepan. Whisk in juices and yolks. Bring to a light boil over medium-high, whisking constantly. Boil until thickened, whisking constantly, about 1 minute. Remove from heat; whisk in butter, zest, and desired amount of food coloring. Pour into prepared crust, and chill at least 4 hours or up to overnight.
Prepare the whipped cream: Beat cream and powdered sugar with an electric mixer fitted with a whisk attachment on medium-high speed until soft peaks form, 2 to 3 minutes. Serve with pie.
