How do you get an island dragon?

[Morbsy]

By breeding a rainbow and poison dragon of course!

How does that even happen? I wanted a double rainbow dragon and I get an island dragon. I can't even keep up with how many dragons there are because I never get any that I actually need. It's frustrating.

[sfallday]

Rainbow is blue and yellow poison is green and purple.
Island blue, from rainbow and green, from poison.