Set your oven to 350°F (or follow the cake mix directions if different).
Prepare the cake mix according to the package instructions using the water, eggs, and oil. Pour the batter into a greased 9x13-inch baking dish and bake for 22–25 minutes, or as directed. Let the cake cool completely.
While the cake is cooling, prepare the red and blue Jell-O in two separate bowls.
For each: Dissolve the Jell-O powder in 1 cup boiling water. Stir in 1 cup cold water. (Note: You’ll only need 1 cup of each prepared Jell-O for the cake. Save the rest to chill and enjoy later!)
Once the cake is cooled, use the handle of a wooden spoon to poke holes about 1 inch apart all over the cake—about 50 holes. Don’t poke all the way through, just about 3/4 of the way down.
Starting in one corner, carefully pour 1 cup of red Jell-O into every other row of holes at a diagonal angle. Then repeat with 1 cup of blue Jell-O, filling the remaining rows. Work slowly and try not to mix the colors, or you'll end up with purple cake!
Cover the cake and refrigerate for at least 2 hours, or until fully chilled and set. The longer the better! Great to prepare the day before so the Jell-O can fully set.
Spread a layer of whipped topping over the entire cake. Just before serving, decorate with fresh blueberries and sliced strawberries.
Cut into squares and enjoy this colorful, patriotic treat!
