Preheat oven to 350°F. Line bottom and sides of a 13- x 9-inch pan with heavy-duty aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil with cooking spray.
Pulse almonds, 2 cups of the flour, 1/2 cup powdered sugar, and 1 teaspoon of the grapefruit zest in a food processor until almonds are finely chopped and mixture is combined, about 5 pulses. Add butter, and pulse until mixture is crumbly, about 10 pulses. Press mixture evenly onto bottom of prepared pan.
Bake in preheated oven until lightly browned, 18 to 22 minutes.
Meanwhile, whisk eggs in a large bowl until smooth; whisk in granulated sugar, grapefruit juice, lemon juice, and remaining 1 tablespoon grapefruit zest. Stir together baking powder, salt, and the remaining 1/3 cup flour in a small bowl; whisk into egg mixture. Using a small wooden pick, stir a small amount of deep pink food coloring gel into filling mixture. Pour mixture over hot baked crust. Return to oven, and bake until filling is set, 24 to 26 minutes. Cool in pan on a wire rack 30 minutes.
Remove from pan, using foil sides as handles. Cool completely on a wire rack, about 30 minutes. Transfer to a serving dish. Remove foil; cut into bars, and garnish with powdered sugar.