Why shia's wear black in Muharram?

Black in Muharram:

Traditionally, the Shia wear black clothes in the month of Muharram. In fact, during this month, it is considered Mustahab (highly recommended) to do so. We find most Shia do wear black in certain parts of the year, including the Shia priests (scholarship) and Maraje’ (high scholars). In fact, the Shia Ayatollahs tend to wear black year-round, and very rarely do we see them not wearing black cloaks.

Black beside Muharram:

Islam is not a fashion show. Many Muslims today focus on things that are not important, and judge and criticize people for minor things while the world is burning.

Our big problems are dictatorship, economic inequity, racism, classism, consumerism, materialism, shirk through worshipping the ego or false ideologies, poverty, warfare, social breakdown, and environmental degradation. What color clothing people are wearing is not a big problem. It is allowed to wear black clothing or white clothing or any other color clothing, and it is wrong to prohibit the things that Allah has allowed. There are enough haram things.

Their criticism is based on a sense of dislike of the other (a sort of tribalism - those people are different from us and do things differently from us, so they must be wrong), and if they don't criticize this, they will find something else to criticize. This attitude is one of the defects that some human beings have - people often fear or dislike people who are different than themselves and who seem "foreign". It is one of the things that Islam should ideally remove from people, but sometimes there is still progress to be made.

Besides, in places like Saudi Arabia, it is normal for women to wear black outside, and women are half the population, so it is really a strange criticism. 

And, many people say that green was the Prophet's (S) favorite color. This is somewhat beside the point, but just putting it out there. 

Also there is no requirement in Shi'ism to wear black, it is simply a custom that has developed in many places during certain time periods. But Shi'is are religiously allowed to wear any color they want. 

Black Color and History:

 Umm Khaalid bint Khaalid said: “The Prophet (peace and blessings of Allaah be upon him) was brought some clothes, among which was a small black khameesah [garment]. He said, ‘Who do you think we should give this to?’ The people remained silent. Then he said, ‘Bring me Umm Khaalid,’ and she was carried to him. He took the khameesah in his hand and put it on her, and said, ‘May you live long and wear it out.’ There was a green or yellow mark on it, and he said, ‘O Umm Khaalid, this is sunnah (good),’ and sunnah is an Abyssinian word.” (Reported by al-Buhari ). Jabir (may Allah be pleased with him) said: “I saw the Messenger of Allaah (peace and blessings of Allaah be upon him) on the day of the Conquest of Makkah, wearing a black turban.” (Reported by Muslim). ‘Aa’ishah (may Allaah be pleased with her) said: “I made a black burdah (cloak) for the Messenger of Allaah ( ) peace and blessings of Allaah be upon him), and he wore it, but when he sweated in it he detected the smell of wool on it, so he took it off, because he used to like pleasant smells.” (Reported by Abu Dawood. Al-Haakim (4/188) said: it is saheeh according to the conditions of the two shaykhs. Al-Dhahabi agreed with him. Shaykh al-Albaani said in al-Saheehah (5/168, no. 2136): It is as they said. Abu Dawood named a chapter in his Sunan “Bab fill-Suwaad (chapter on black clothes)”. The author of ‘Awn al-Ma’bood (11/126) said: The hadeeth indicates that it is permissible to wear black and that there is nothing makrooh in doing so).

Prohibitions:

Black is permissible for both men and women alike. One of the false innovations (bid) connected to this color is the practice of deliberately wearing black at times of bereavement, which also involves imitating the Christians. Shaykh Muhammad ibn Saalih al-‘Uthaymeen said (Fataawa Islamiyah, 3/313): “Wearing black at times of bereavement is a false symbol that has no basis. At times of bereavement people should do what is commanded in Islam, which is to say ‘Innaa Lillaahi wa inna ilayhi raaji’oon. Allaahumma ajirni fi museebati wa’khluf li khayran minhaa (Truly! To Allaah we belong and truly, to Him we shall return. O Allaah, reward me for my calamity and compensate me with something better than it).’ If a person says this with faith and the hope of reward, Allaah will reward him for that and will replace what he has lost with something better.” He also said: “Allocating certain clothes for mourning is an act of bid’ah (innovation) in our opinion, and because it could indicate that a person is discontent with the decree of Allaah.”