Buy shortcakes that come about four to a package...or can make your own...i.e. like strawberry shortcake cakes
Layer, blueberries, strawberries/raspberries, cool whip, another short cake & build them as high as you like. Top with cool whip and berries. Could replace the cool whip with ice cream of choice. Could sprinkle in chocolate chips.
Parfaits, vanilla pudding, red berries, pudding, blue berries, etc., top with cool whip...same for a parfait using crumbled spongecake & layring berries pudding or cool whip. Be as creative as you like. Sure you'll come up with many ideas. Put a flag on top.
Also lots of food colorings out there.