I don't know if it's all ready been said, (i'm lazy and didn't read through the whole thread, sorry) but i think making the rainbow vertically orientated, and like the Jadeite set and some other "gradient" sets might look better, and be a little bit more unique, all though it's probably been done before too.
You could make different sets, where the last/main color is different. So, like the jadite, or a better comparison would probably be the 1976 set, but replace the brown/white with whatever color you want to be last in the rainbow (i say purple, or blue, becasue yellow is the worst color ever..)