Bukhari

Hadith Commentary:
In the narration of Yunus, there is an additional detail that he had applied henna (mehndi) and katam (wasm) as dye.
The narration of Imam Ahmad also mentions this similarly, but Imam Muslim has narrated from Anas radi Allahu anhu that the Messenger of Allah sallallahu alayhi wa sallam did not dye his hair; rather, Abu Bakr and Umar radi Allahu anhuma did dye their hair. He says that the hair of the Messenger of Allah sallallahu alayhi wa sallam appeared reddish because he used to apply yellow perfume to them.
(Wahidi)

Hadith Commentary:
(1)
The original incident is as follows: Umm Salamah radi Allahu anha had some blessed hair (mubarak hair) of the Messenger of Allah sallallahu alayhi wa sallam, which she kept in a small silver container. When someone would fall ill or be afflicted by the evil eye, they would send a vessel of water to Umm Salamah radi Allahu anha. She would place the blessed hair in it and shake the vessel, and then the water would be given to the sick person to drink. By the command of Allah Ta'ala, the person would become healthy.

(2)
Indeed, the blessed hair is a means of attaining blessing (barakah), but the belief must be that the blessing is from Allah Ta'ala and comes by His command; nothing happens without His permission. In one narration, it is mentioned that Umm Salamah radi Allahu anha had reddish hair of the Messenger of Allah sallallahu alayhi wa sallam, on which there was a mixture of henna and katam dye. In some narrations, it is mentioned that the hair became red due to the application of perfume.

(3)
Hafiz Ibn Hajar rahimahullah writes that when hair is separated from the body and a long time passes, its blackness naturally turns into redness. It is possible that the hair with Umm Salamah radi Allahu anha became red due to being kept for a long time. (Fath al-Bari: 10/434) Nowadays, news about the blessed hair of the Messenger of Allah sallallahu alayhi wa sallam is published in newspapers; according to our inclination, all these are mere displays and a means of gaining cheap fame. Our detailed fatwa regarding the blessed hair can be seen in Fatawa Ashab al-Hadith: 1/42-48. And Allah knows best.