First, bake the white cake mix as described on the box to make 2 rounded 9'' cake
Put a parchment paper on two large plates. When you take out both cakes that are baked, place them on each plate and let them cool for 15 minutes at room temperature.
In the meantime, pour the Jello powder into 2 medium bowls (one for each flavour).
Add 1.5 cups of boiling water to each bowl of Jello powder and slowly stir until the powder is all dissolved. About 3 minutes each and put aside.
Take a fork and poke holes gently into both cakes; about 2cm deep. Don't go too deep, as you don't want to break your cake. Poke about 12-16 times on both cakes.
Now, take a spoon and pour the liquid Jell-O gently over the top of each cake. Make one cake a flavour (colour) and the other a different flavour (colour).
Put the 2 cakes in the fridge for 15 minutes.
Take the 2 cakes out of the fridge and gently turn one cake over the other. Be careful as the cake is full of liquid and will be heavier and easy to break.
Now put the Cake in the refrigerator for 8 to 24 hours.