Make cake in a 9×13 inch baking pan, according to the instructions on the back of the box.
15.25 ounce box white cake mix, Ingredients needed to make cake like eggs
After baking and while cake is still warm, use a thick skewer or back of a wooden spoon to poke holes in the top of the cake.
Allow cake to cool completely. Remove cake from the pan and place on a large serving platter.
Stir together red jell-o and ¾ cup boiling water until the jell-o has dissolved.
3 ounce package red jell-o
Stir in ½ cup cold water.
Pour red jell-o mixture in every other row of holes.
Stir together the blue jell-o and remaining ¾ cup boiling water in a separate small bowl until jell-o has dissolved.
3 ounce package blue jell-o
Stir in ½ cup cold water until combined.
Whisk boiling water and gelatin together until fully dissolved. Pour over the top of the cake.
Place cake in the fridge to set for at least 4 hours.
Spread thawed whipped topping over top of the cake. Decorate with sprinkles.
8 ounce container frozen whipped topping thawed or homemade whipped cream, Red white and blue sprinkles
Notes
Cake mix: You can substitute a homemade white cake for the cake mix if you prefer. Just be sure to use white cake as yellow won't give you the same color effect.
Red Jell-O: You can use any flavor of red Jell-o that you like.
Blue Jell-O: Similarly, you can use any flavor of blue Jell-O.
Whipped topping: Whipped topping, like Cool Whip or TruWhip, or homemade whipped cream will work. Homemade whipped cream, while delicious, doesn't hold up as well as whipped topping. You could also use whipped cream frosting if you prefer.