From most sources people say it's completely random, that you can get a rainbow dragon anytime you breen any kind of dragons, it's just really rare. It's not impossible as i have quite a few friends who have one and even a few who have two. Some sources say certain breeds have better chances at breeding a rainbow dragon and some say that having the rainbow flags around your park make the chance of breeding a rainbow dragon better. Personally after I get most of the breeds I'm just gonna breed a fire and plant dragon until I give up or a rainbow dragon egg appears because either the egg they make will be only 30mins or 60mins which are the two lowest incubation egg times out of all the mixed dragon breeds. --- I refuse to have a signature... wait a second!