You can breed the Opal dragon with this combination: Lava Dragon + Mud Dragon. It's only available for the month of October.
"Opal dragons are said to be capable of displaying every color in the visible spectrum. In fact they're even supposed to be able to exhibit some invisible ones as well! Some say that invisible dragons are really just opal dragons parading around in these...harder to see colors. Our wizards can neither confirm nor deny this rumor."