Shama-il Tirmidhi Chapter 5, Hadith Number 7 (42).

Abu Rimtha Taymi R.A. says: “I went with my son to Rasulullah (Sallallahu alaihe wasallam). (Probably he had not see Rasulullah (Sallallahu alaihe wasallam) before). When I saw him I said to myself immediately, ‘This is the true Messenger of Allah’. At that time Rasulullah (Sallallahu alaihe wasallam) was wearing two pieces of green clothing. (His lungi-sarong, waist-wrap and shawl were both green). On a few hairs there were signs of old age, but those hair were red”.

Commentary.
The signs of dignity, ‘Anwaar-e- Nubuwwah’ (prophetic illumination) and ‘Aathari-Haybah’ (signs of awe) were thus seen on the blessed face of Rasulullah (Sallallahu alaihe wasallam).

When one sees these signs, such words naturally come to one’s tongue: “There is no doubt he is Allah’s Messenger and cannot be false.” The ahaadith state that many Sahabah R.A. uttered such words when they first met Rasulullah (Sallallahu alaihe wasallam). The above Hadith also states that the hair of Rasulullah (Sallallahu alaihe wasallam) was red. The ulama disagree whether Rasulullah (Sallallahu alaihe wasallam) dyed his hair or not. Some are of the opinion that Rasulullah (Sallallahu alaihe wasallam) dyed his hair and they base their argument on the red hair. Others deny this and say that before the hair becomes white, it usually turns red. The redness was natural, and not that of a dye.