Prepare cake mix according to directions on the box. Pour into a greased 9×13-inch pan and bake according to package directions. (Should be something like 350˚F for 25-35 minutes.) Allow the cake to cool for 15 minutes; it should be warm still. Poke holes in rows about an inch apart across the cake with the end of a wooden spoon.
Mix red gelatin with ½ cup boiling water until dissolved. Add ½ cup cold water and stir. Pour red gelatin into half of the holes.
Mix blue gelatin with ½ cup boiling water until dissolved. Add ½ cup cold water and stir. Pour blue gelatin into the remaining holes. Try not to mix the colors. Cover and refrigerate 4 hours or overnight to let the jello set.
Before serving, spread whipped topping evenly over the cake, then top with patriotic sprinkles (affiliate). Cut into 15 pieces (3 rows by 5 rows). Use a serving spatula to lift the slices of cake out of the pan.
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Notes
To Store: Tightly cover the cake with plastic wrap and store in the refrigerator for up to 3 days, or wrap it again with foil and freeze for up to 3 months (preferably without sprinkles, as they may bleed into the whipped topping once thawed).