Iormúngr was a male dragon. He was bound to a Rider, but fathered Saphira II with a wild dragon named Vervada. Saphira describes him as one of the best dragons, and is delighted to have him as a father. Iormúngr was killed in the Fall of the Riders, along with Vervada. Strangely, very little is known about Iormúngr, not even the color of his scales or the name of the Rider he was bonded to.
When asked what color Iormúngr and Vervada were in a Q&A session, Christopher Paolini said, "I've yet to decide that, actually, but with a name like Vervada, I wouldn't be surprised if Saphira's mother was crimson."
As Vervada, Saphira's mother, was mostly likely "crimson", Iormúngr may have been a blue color, like his daughter, Saphira II.