You will need:
A large tub of sour cream
6 small boxes of Jello ( All different colors)
A day where you are going to be home most of the day.
Dissolve your first box of Jello in 1 cup of boiling water. Separate the Jello in half, into two bowls. Add two tablespoons of water to one and then put that bowl of Jello that you added the water to, into a clear 9x13 pan. Place the pan flat in the fridge and allow to set for 30 minutes.
After 20 minutes or so, add 1/3 of a cup of sour cream to the other bowl of Jello that has been sitting on your counter. Mix until well combined. After the 30 minutes have passed and the Jello in the fridge has set up, pull the pan of Jello out and pout the bowl of Jello with the sour cream in it over top the set Jello. Place back in the fridge for 30 minutes.
Repeat until you have all 12 layers. I have made it with 10 layers, such as in the picture above which works too. Make sure the pan is as flat as possible when you put it in the fridge so your rainbow doesn't turn out crooked.
This Jello recipe takes quite a bit of time but the Ooo and Aahhhs are worth it!