All Sikh Gurus since Guru Nanak Dev Ji wore turban. However, the covering of hair with turban was made official by Guru Gobind Singh Ji, the tenth Guru of Sikhs. The main reasons to wear turban is to take care of the hair, promote equality, and preserve the Sikh identity.
Sikhs do not cut their hair as a respect towards God and the turban protects the hair from dust. Some people might ask the question that if Sikhs cover their hair on head, why don’t they cover their beard? Dirt is not a big problem for beard. The beard can be cleaned very easily while washing the face. Covering of the head by turban also symbolizes respect towards God. People form many other religions including Hindus, Jews, and traditionally many Christians including Catholics cover their head while visiting their church (Source: Christian Head Covering). Since Sikhs believe God to be present everywhere, they cover their head not just in church but everywhere else as well.
Sikhs are only 2% of India’s population, turban also helps to identify Sikhs. Upon establishing turban as a Sikh identity Guru Gobind Singh Ji said, “My Sikh will be recognized among millions”.
In addition, the people from high class used to wear turban and many high status individuals still wear turban. The high class turban wearing individuals were called Sardars (leaders). This segregated people from high and low classes. So in order to eliminate the class system associated with turban, Guru Gobind Singh Ji made each and every Sikh a Sardar. He also rejected class system by giving all Sikh the last name Singh and Kaur.
Many people mistake Sikhs and look at them as Muslims due to their turban. Turban is used as a head covering by many cultures. Many cultures in Africa wear turban as well. Only the high officials in Islam wear turban. Many Hindu priests wear turban as well. Identifying people wearing turban and looking at them as terrorist and making them a target of hatred is completely wrong. Just like every person wearing a baseball hat is not a bank robber; every person wearing a turban is not a terrorist. If you see a man walking down on the streets of western countries wearing a turban, there is 99.9% chance that he is a Sikh since Muslims and Hindus in western countries do not wear turban.
Sikhs do not usually wear red or green. The most common colors worn by Sikhs are white, blue and black. The old and/or spiritual people usually wear white. Black and blue are the most common colors worn by the youth. The main purpose of the head covering is to cover the head and the hair. The choice of colors is totally up to the individual.