Whip the heavy whipping cream in an electric mixer until stiff peaks form, then transfer it to another bowl and set it aside. The cream should stand up when the beaters lift, looking smooth and fluffy rather than loose or grainy.

Beat the lemon pie filling and cream cheese in the mixer until fully smooth before adding the vanilla extract, yellow food coloring, if using, and powdered sugar. When it is ready, the mixture should look glossy and thick, with no small cream cheese lumps hiding in the bowl.

Fold the whipped cream into the cream cheese mixture in batches with a rubber spatula until no streaks remain. Use broad, gentle turns so the mousse stays light and cloud-like instead of becoming flat and heavy.

Stir together the graham cracker crumbs, sugar, and melted butter or margarine until the crumbs are evenly coated. The mixture should feel like damp sand, and the butter should be warm at most, not hot, before it goes near the cold mousse.

Spoon a small layer of crumbs into the bottom of each cup, pipe in the mousse, and finish with more crumbs on top before chilling to serve. The layers should look neat and hold their shape, with the mousse sitting high and creamy rather than slumping.