(loved those Capital One commercials ...)
Gary, since you gave the answer you did above, I wish to ask three questions I would not have otherwise asked.
We know of the Prismatic Spells: Prismatic Spray (7th), Prismatic Wall (8th), and Prismatic Sphere (9th.)
In Dragon Magazine, they introduced Jaran's Prismatic Sword. It was 9th level. I disagreed with that assessment, thinking it should have been higher. What would it have been in your game?
And (rather obviously) Prismatic Plate Armor (or, just plain Prismatic Armor) comes to mind.
What level would it be?
If there is Prismatic Sword, perhaps there is Prismatic Bow (and Arrows) or Prismatic Bolts (1 bolt per level!) What level would this spell be?
Incidentally, in 2nd Edition they created spells of 10th, 11th, and 12th level. But the increase in spell power was not linear. It was exponential.
In the case of the above questions, I am assuming (hopefully correctly) you go by a linear approach, so no analogy to the 2nd edition spells is drawn here.