With the highest income of any dragon and a habitat that can store a small fortune, the Rainbow Dragon is the ultimate Dragonvale Dragon — if you can get it! It's quite possibly also the hardest dragon to breed in Dragonvale. No particular combination of elements is required except that need four different elements in the breeding combination and a lot of luck. Most users requires many tries to get the dragon. Having a existing Rainbow habitat and breeding high-level dragons seems to help.
"Normal words cannot begin to describe the resplendence of these, rarest of all dragons. The first person to ever witness one stared for hours before running home to send a poet to describe the beast. When the poet arrived, the dragon had left without a trace, leading many to believe that the explorer made up the whole thing. Only now do we realize he wasn't crazy at all."