This thread
http://elementscommunity.org/forum/index.php/topic,27926.msg449817.html#msg449817has a rainbow deck that I used to use a lot for FGs and arena. It's an older deck, though, and was not designed to combat the newer shards, which can really wreak havoc with it. I have a couple more modern variations on my home computer that I can post in a few hours for you. I thought of them specifically because of this:
I also really enjoy playing rainbow decks, I love the versatility and fun. I like decks that are hard to play, but are good when you play them right.
It's pretty much the exact definition of my 60 card "The Deck".
edit:
This one in my notes mentions it was built for elements 1.30. If memory serves, at that point there were only 6 shards. (gratitude, divinity, readiness, serendipity, void, sacrifice) Shard of focus tends to rip this deck a new one, due to the heavy permanent reliance, but then, that card wrecks almost everything :p
A week or two ago I built a nymph version mod for it, and it works fairly well, too, but odds are that even if you have a lot of nymphs, you might not have the same ones, so it will be quite hard to build. Still, I'll post it in case it gives you some ideas.
This one's another one of my favourites for the arena, just because it's a different deck every time. It uses shard of serendipity (and one mindgate) as the only way to deal any damage, so sometimes you get a fantastic game and sometimes you can actually deck out because you didn't draw enough creatures.
For speedbows, this one probably does good against lower level arena decks, but it likely will fail to platinum and probably gold, too. It does well in pvp, and I specifically designed it for league play and to be able to deal with common decks that appear there.
This is another speedbow I built for use for pvp, but it assumes that both players have agreed not to use shards, so it lacks both my shards and any cards that I put in specifically to deal with shards. I'm assuming that bronze/silver decks don't have many shards, so it might work just as well or better there, but it will probably get owned in gold/plat, where shards are more common.
