Actually we have a full array of color orbs, including "rainbow". People put them on their thread instead of some of the RP tags. It's just decoration.
I like to pick the colors when the thread already has a self-explanatory suffix. ^~^
They don't do anything special. You'll see the option for them if you create a thread.
Thanks for the information! And if those orbs really predicted the future, I would put a rainbow orb on every thread i post just so I could see a double rainbow. :3