Preheat the oven to 350 degrees F (175 degrees C).
Blend together flour, butter, and sugar using an electric mixer in a medium-sized bowl. Press shortbread mixture into the bottom of an ungreased 9x9-inch baking pan.
Bake in the preheated oven until firm and golden brown, 15 to 20 minutes.
Place cold water in a bowl to prepare raspberry filling. Sprinkle gelatin over water; stir. Let stand to soften, about 1 minute.
Puree raspberries using a food processor. Press berry puree through a fine mesh sieve into a medium saucepan; discard the seeds. Stir in gelatin mixture, sugar, and lemon juice.
Bring mixture to a boil over medium-high heat. Reduce heat to medium and cook, stirring frequently while skimming foam from sides of saucepan using a slotted spoon, about 5 minutes. Reduce heat to medium-low. Cook, stirring frequently until bright in color, thickened, and reduced by half, 8 to 10 minutes.
Pour raspberry filling over shortbread and let cool completely, about 2 hours.
Place cold water for lemon mousse in another bowl. Sprinkle gelatin over water; stir. Let stand to soften, about 1 minute. Add boiling water; stir until gelatin is completely dissolved. Mixture will be clear.
Combine heavy cream, sugar, lemon juice, lemon extract, and yellow food coloring in a large, cold, mixing bowl. Beat mixture at medium speed with an electric mixer until stiff peaks form. Pour in gelatin mixture and beat until well blended. Pour lemon mousse over firm raspberry filling and spread to cover. Allow to set completely, about 4 hours, before cutting into bars.
