Prepare and bake the white cake according to the package directions in a 9×13 inch pan. Let the cake cool completely.
While the cake cools, prepare the red gelatin. In a small bowl, dissolve the red gelatin powder in 1 cup of boiling water. Stir until completely dissolved. Add 1/2 cup of cold water. Set aside to cool to room temperature, but do not let it set.
Prepare the green gelatin the same way using the green powder, 1 cup of boiling water, and 1/2 cup of cold water. Set aside to cool to room temperature.
Once the cake is cool, use the handle of a wooden spoon or a large fork to poke holes all over the surface of the cake, about 1 inch apart.
Carefully pour the cooled red gelatin mixture evenly over the entire cake, allowing it to seep into the holes. Refrigerate the cake for about 1 hour, or until the red layer is partially set but still sticky.
Repeat the poking process over the red layer. Pour the cooled green gelatin mixture evenly over the cake.
Refrigerate the cake for at least 4 hours, or preferably overnight, until both gelatin layers are fully set.
Prepare the topping by mixing the thawed whipped topping, sweetened condensed milk, and vanilla extract in a bowl until smooth.
Spread the prepared topping evenly over the set gelatin layers.
Garnish the top with red and green sprinkles before serving.
Notes
For easier gelatin pouring, use a measuring cup with a spout.
You can use store-bought cool whip or make your own stabilized whipped cream for the topping.
Chill the cake thoroughly before slicing to ensure clean cuts.