User blog comment:FairyRaven/Fairy Tail Chapter 325: Unity, Review and Commentary/@comment-49.176.34.54-20130330054337/@comment-7418318-20130330083646

Zirconis was dead for more than 400 years (as opposed to the known "Dragon Slayer teachers" who vanished 14 years before the GMG) and juding just from hair color alone is a mighy stretch.

In the Fairy Tail world green hair doesn't seem to be all that unusual. Bisca, Freed, Sol, Karen Lilica and Karacka all have green hair and non of them is a Dragon Slayer.

On top of that, from the flashbacks we know that both Laxus and Cobra already had their respective hair colors even before they were implanted with Dragon Lacrima.