Many different societies and groups of people, including those in today’s civilized world, use signs and symbols to recognize and communicate with one another. Seyyids, as a result of historical occurrences and religious and social motives have also taken on different signs and slogans. From a religious point of view, seyyids wear black turbans in following the practice of their noble ancestor, the Holy Prophet (pbuh). There are many narrations which describe him wearing a black turban. Furthermore, it has been narrated that the Prophet (saws) also wore other color turbans, in particular the color white.
Many different societies and groups of people, including those in today’s civilized world, use signs and symbols to recognize and communicate with one another. For example: Baloochis and Kurds (cultural denominations within Iran) have their own respective attire, the Ahlul-Haqq keep mustaches, Mowlavis in Iran wear unique clothing and keep long beards, Arabs wear their own traditional clothing, followers of the Sikh faith in India wear turbans and keep mustaches, etc.
Seyyids, as a result of historical occurrences and religious and social motives have also followed and used different signs and slogans. From a religious point of view, seyyids wear black turbans in following the practice of their noble ancestor, the Holy Prophet (pbuh). There are many narrations which describe him as wearing a black turban. Shamsudeen Zahabi (a prominent Sunni scholar), has mentioned five narrations found in the three primary Saheeh books of narrations and various others which describe the Prophet’s turban (pbuh) as being black. For the sake of being concise, two of those narrations should be sufficient for establishing a position in this matter.
1- Jaber narrates that on the day of the conquer of Makkah, the Prophet (saws) entered the city wearing a black turban on his head. 
2- Jafar bin Amr bin Harith narrates: “I saw the Prophet (saws) on the pulpit wearing a black turban and a portion of the turban was hanging between his two shoulders.”
Furthermore, wearing a black turban was customary amongst Arabs at the time. For example, Ali bin Abi Talib (as), Abd al-Rahman bin Aouf, Saeed bin Maseeb, Umar, Muawiyah, Abu Musa Ash’ari, Muhammad bin Hanifah, and many others all wore black turbans.
Since black was a sign of the Abbasids during their reign, the majority of Muslims were forced into wearing identically colored turbans as their rulers. The Imams (as) were in opposition to this and only wore black when forced to as a result of taqiyah (dissimulation/concealing one’s faith). Davood Raqee narrates: “The Shia always questioned Imam Sadiq (as) regarding wearing black clothing. Once, I saw the Imam (as) sitting while clothed in a black jubbah (loose-fitting clothing that is worn over the rest of one’s clothes), a black hat, and a pair of black khaf (leather socks) with black cotton lining. He said: ‘Render your heart white and you may wear whatever you please.’ Saduq, a prominent Shia narrator of hadith explains: ‘The Imam performed this action out of compulsion and taqiyah. He said this as a result of accusations that had been made against him on behalf of the enemy, for the Imam does not consider the wearing of black clothes to be permissible.’” 
Ibn Anbah writes that Seyyid Radhiyy was the first Talebiyy that used the color black as a sign. “و هو اول طالبی جعل علیه السواد”. From this passage we understand that before Seyyid Razi, seyyeds did not use the color black as a sign or symbol. Thus, Seyyed Radhiyy was the initiator of using the color black as a symbol.
Only after Seyyed Radhiyy, did using the color black amongst seyyids and those from the tribe of Bani Hashem become customary. Seyyed Radhiyy, was born in the year 359 and died in the year 406 (hijri); meaning that the use of this symbol amongst seyyids became common sometime around the fourth century. Before the time of Seyyed Radhiyy, seyyids did not associate themselves with the color black. As mentioned, one of the reasons for this was their movement against Bani Abbas who had adopted the color black as a sign for themselves. This resulted in narrations regarding the wearing black clothes as being makruh.
Also, it has been narrated that the Prophet (pbuh) wore different colors of turbans. One example is a white colored turban which was generally referred to as Sahab (cloud). In history, we find significant figures who lived at the advent of Islam as wearing white turbans. For example: Ali ibn Husein (as) (Imam Sajjad), Salem bin Abdullah, Saeed bin Jabeer, and Kharjeh bin Zayd all wore white colored turbans.
During the time of Imam Reza (as), green was a symbolic color amongst the Alawiyys (followers of the Imams), and thus wearing a green turban became widespread.
Some deem that kings of the Safavi dynasty in Iran, designated the use of black turbans amongst seyyids as to continually symbolize and mourn oppression that occurred against their ancestor, Imam Hussein (as). The problem with this opinion is that this claim does not correspond with the cultural norms of the Iranian people; for during the time of mourning they wear black clothes, not black turbans.
 Shamsuddin Dhahabi, Seyr A’lamul-Nubala’, vol. 1, pg. 372.
 Seyr A’lamul-Nubala’, vol. 1, pg. 372; Siratu Ibn Kathir, vol. 4, pg. 78 “عن جابربن عبداللَّه. ان النبی(ص) دخل مکة یوم الفتح و علیه عمامة سوداء”
 Ibid and Siratu Ibn Kathir, vol. 4, pg. 708, the chapter on the prophet’s traditions: “عن جعفر بن عمرو بن حریث عن ابیه: رأیت النبی(ص) علی المنبر و عیه عمامة سوداء قد ارخی طرفها بین کتفیه”
 Wasa’ilul-Shia, vol. 4, chapter 19, hadith 5469: “کانت الشیعة تسأل اباعبداللَّه(ع) عن لبس السواد قال فوجدناه قاعدا علیه جبة سوداء و قلنسوة سوداء و خف اسود مبطن سبواد... ثم قال: بیض قلبک والبس ما شئت، قال الصدوق: فعل ذلک کله تقیة لانه کان متهما عند الاعداء بانه لا یری لبس السواد...”
 Umdatul-Taleb; Mu’jam Rijalil-Hadith (Ayatullah Khu’i), vol. 16, pg. 20.
 Abdul-Husein Ahmad Amini Najafi, Al-Ghadir fil-Kitabi wal-Sunnah wal-Adab, vol. 3, pp. 290-293.
 With help from http://www.ichodoc.ir/p-a/CHANGED/157/HTML/157_8.htm
 Ali Asghar Faqihi, Tarikhe Mazhabiye Qom, pg. 115.