I’m having an outdoor, rainbow themed wedding near a lake in the spring of 2011. What are fun decor ideas?

I’m trying to come up with some ideas that are fun and colorful, yet tasteful. I want my wedding to stand out in my loved ones minds and be as special to my guests as it will be to me. Bride’s maids will wear the same dress with my maid of honor in red, the next in and orange/yellow dress, then green, blue, and finally purple. What are some tasteful ideas for wedding and reception decor? What is a good flower for the occasion? Also, I need flower arrangement ideas. I’d like to play with prisms and light somehow, too. Ideas, anyone?