I picked up a 6oz box of blue, yellow and red jello plus a box of knox gelatin. To get these cool layers, mix the blue jello with an envelope of the knox gelatin. Pour in two cups of boiling water. Dissolve the gelatin. Add 1/2 cup of ice. Stir until thick. Remove any remaining ice cubes and any bubbles that may have formed. Pour into a 9×13 inch pan. Let set in the refrigerator.Repeat with the yellow jello (+ packet of knox) and pour it over the set blue. Repeat with the red jello (+ packet of knox) and pour it over the yellow. And there you have it, Rainbow Jello.
Oh, and these can be made and cut a day ahead.