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1
Place lime, orange, and strawberry gelatin mixes into 3 separate 9x9-inch glass dishes.
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2
Pour 1 cup boiling water into each flavor of gelatin mix; stir the gelatin until dissolved.
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3
Pour 1 cup cold water into each flavor of gelatin, stir to combine, and refrigerate the 3 flavors of gelatin until set, at least 4 hours.
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4
Heat pineapple juice in a saucepan until hot but not simmering. Stir unflavored gelatin into pineapple juice until dissolved. Refrigerate pineapple juice mixture until cool and thickened, about 30 minutes. Stir occasionally.
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5
Mix graham cracker crumbs, melted butter, and brown sugar together in a bowl. Set 1 cup of crumb mixture aside for garnish. Press remaining 2 cups crumb mixture into the bottom of a 9x13-inch cake pan to make a crust.
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6
Pour cream into a large metal mixing bowl and beat with an electric mixer until the cream is fluffy and forms soft peaks, about 3 minutes. Slowly beat sugar into whipped cream, 1 or 2 tablespoons at a time; beat in vanilla extract.
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7
Cut the different flavors of gelatin into cubes. Use a thin spatula to lift the cubes away from the bottom of the dish and loosen them.
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8
Very gently fold the thickened pineapple juice and gelatin cubes into the whipped cream mixture until thoroughly combined. Spoon into the graham cracker crust.
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9
Sprinkle the cake with reserved graham cracker crumb mixture.
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10
Refrigerate until set, at least 3 hours to overnight.
