Normally, I would find it odd that he left his hair partially white for 2 years without changing its color artificially, only to dye it once he regained his memories. It seemed to me Haise wasn't the type of person to worry about his hair color, much less Kaneki. However, seeing Kaneki with glasses, gloves, and a nice black overcoat suggests that he suddenly has a budding interest in ghoul investigator fashion or that he wants to appear more professional or human in front of the other ghoul investigators (may be related to the very possible speculation that he wants to get a higher position within the CCG for more influence within the organization). So yes, it is possible that he just went ahead and dyed it too.