Prepare and bake the white cake according to package instructions in a 9x13-inch pan.
Allow the cake to cool for about 10–15 minutes.
Using the handle of a wooden spoon, poke holes all over the cake.
In separate bowls, dissolve each gelatin flavor with 1 cup boiling water, then add ½ cup cold water to each.
Carefully pour the red gelatin over half of the cake and the blue gelatin over the other half, letting it soak into the holes.
Refrigerate the cake for at least 2–3 hours until fully set.
Spread the whipped topping evenly over the chilled cake.
Garnish with fresh strawberries and blueberries if desired.
Slice, serve, and enjoy.