Usually, the Shoyukh (Shaikhs) or Aalims (scholars) of Shia put an Ammamah or turban on their heads, what is the purpose of this act? And why some of them wear it in black color and many of them wear white Ammamah?
Initially, I ought to mention that Ammameh or turban is a cover for the head of men (Arabs) which consists of a thin cloth which is wrapped around the head(or actually on the head). After the appearance of Islam, it became a common issue across the Islamic countries. At first, scholars used it and afterwards other people used it.
Of course the apostle of Allah(pbuh) had Ammameh as well. His Ammamah was called Sahab. in accordance with some traditions, the prophet (pbuh) granted his turban to Imam Ali (a.s.). (Note: there are different opinion concerning its shape and color).
According to Shia:
Nowadays, Ammamah is used just for a specific group of people. In truth, As well as Scholars, it is put on the heads of the students of Howzah-Elmiyah who pass some specific terms in Howza(Al-Elmiah)and in fact they read different Islamic issues, afterwards will be able to put Ammamah on their heads.
In fact, it is the symbol of Prophet Muhammad (S) and 12 Imams and it likewise shows that for example the person who has worn an Ammamah, he is a Shia Shaikh and actually he could be as an agent of the Prophet (pbuh) and 12 imams and people can ask them (their Islamic questions).
Usually, there are two kinds of Ammamah, one of them is black and the other one is white. As you know, Sadat or Seyyeds who their origins come back to the prophet Muhammad (s) (or 12 Imams),they wear black Ammamah, but other shaikhs wear white Ammamah. (Note: green Ammamah is related to Sadat as well)
Read the following traditions regarding black Ammamah;
Imam Sadiq (a.s.):
The prophet (pbuh) had a black Ammamah on his head on the day in which he(pbuh) conquered Mecca, and entered Masjedul-Haraam.
Vasaaelul-Shi’a, Vol.3, Pg.379, H10, Baab30
Or Abdollah ibn Solaiman quoted from his father:
Imam zain-Al-Abedin(imam-Sajad) a.s. entered the mosque while had a black Ammamah on his head.
(Vasaaelul-Shi’a, Vol.3, Pg.378, H9)
According to the mentioned traditions, Sadat put black Ammamahs on their heads and this is the Sunnah of the Prophet(pbuh) and Imams(a.s.). So, the other Shaikhs who are not Seyyed (their origin and descendant don’t related to the Prophet (S), wear white Ammamah.
I think this is a following of special Prophet Mohammad's clothing. Specially sheikhs try to be just like dear Prophet, even in appearance and clothing.