If he was so concerned about her hair, Celestia feared for his sanity if he ever discovered Rainbow Dash! Young she may have been, for a therapist of her reputation, but the dreams had imparted a certain sense of wisdom to her... perhaps being a thousand-odd year old alicorn had its benefits.
She gladly shook his hand, never more thankful that she was always warm. "The hair colour is largely natural, in case you were wondering. Side effect of those dreams."