One point I picked up from another blogger was that her red and orange layers bled so closely in color that she was not able to tell them apart. So to set my red apart, I added a few drops of red food coloring before pouring to make sure the red was deep and vibrant.
When all the layers are set and all your little leprechauns are ready for dessert, top each cup with a big whipped cream cloud and for a wee bit of extra charm, tuck in a chocolate gold coin. As an alternative, for those with milk allergies or lactose-intolerance issues, you can use mini marshmallows or Fluff to top your rainbows instead.
FYI: These five boxes of jello mix filled nine 9 oz. cups to the BRIM. I am sure you could fill ten if need be, but the layers would be thinner of course.