Azhar means that he had a white complexion mixed with redness; Ibn Hajar said, “Azhar means white imbued with redness. This was explicitly mentioned in another hadeeth by Anas as well, which was reported by Muslim, Sa’eed ibn Mansoor, At-Tayaalisi, At-Tirmithi and Al-Haakim, and it was narrated by ‘Ali, who said, ‘The Prophet (???) was white with his whiteness imbued with redness.’”
He also said, “What is meant is that he was not extremely white and not extremely dark, but his whiteness was mixed with redness.”