Jaina Proudmoore, who is not only the founder and leader of the Theramore Isle, but also the most powerful human sorceress on Azeroth, will appear in Mists of Pandaria with white hair instead of the usual blonde. This was reported by Wowhead.com after a datamining of the latest beta patch.
Robin Torres at WoW Insider predicts that the color change is due to the traumatic events set to happen at the Battle for Theramore and not to age. The model also show a much more tattered set of robes compared to earlier 3D models.
The changes also matches the previous released artwork and the book on Jaina Proudmoore coming out this summer.
Source: WoW Insider, Wowhead.com