In two small bowls, combine each gelatin flavor with 1 cup boiling water. Stir 1/2 cup cold water into each. Pour each into an ungreased 9-inch square pan. Refrigerate for 1 hour or until set.
Make Lemon Syrup: In a small bowl, combine ¼ cup water, 2 T sugar and 2 T lemon juice and stir until sugar is dissolved. Set aside.
When gelatin has set, cut into 1-inch cubes.
In a trifle bowl or serving dish, layer the strawberry gelatin, then half of the bread cubes. Brush the bread with some of the lemon syrup. Top with half of blueberries and half of the whipped topping.
Then, layer the blue gelatin and remaining bread cubes, brushed with lemon syrup. Top with half of the strawberries and the remaining whipped topping.
Decorate with the remaining berries, arranged in a flag pattern. Sprinkle with powdered sugar, if desired. Serve immediately.