It is not a condition for the hijab to be black. Please give the authentic hadith that proves that women wore colours and not just black during the time of the Prophet, sallallaahu ‘alayhi wa sallam, and that there is no obligation to wear black at all times. I prefer all the hadiths with references in one mail for easy reference and to tell others. May Allah reward you well.
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The basic principle is that any clothes that fulfill the conditions of the hijaab are considered a legitimate hijaab. The texts that underline the obligation of the hijaab have a general indication, and there is no color specified in them. The one who claims that there is a specified color that the Muslim woman must wear, like black for instance, is the one required to bring forth evidence supportive of this claim. One of the pieces of evidence that have a general indication is the verse that reads (what means): {O Prophet, tell your wives and your daughters and the women of the believers to bring down over themselves (part) of their outer garments...} [Quran 33:59]
After a careful search, we could not find any ahaadeeth specifying the color that the female Companions used to wear. The Hadeeth scholars held different views regarding the interpretation of the word Mirt (veiling sheet or mantle) used in the following hadeeth: ‘Aa'ishah said, "The Prophet, sallallaahu ‘alayhi wa sallam, used to offer the Fajr prayer and some believing women used to attend the Fajr prayer with him covered with their Mirt, and then they would return to their homes unrecognized." [Al-Bukhaari and Muslim]
Some scholars interpreted the word Mirt to mean covers made of black hair, and others held that only green covers are called Mirt, and only women wore them. Some other scholars maintained that a Mirt was a cover made of wool or silk or linen. These interpretations and others were cited by Az-Zarqaani in his commentary on Al-Muwatta'. This indicates that Muslim women are allowed to wear different colors other than black.
Muslim scholars established that there is no specific color for the dress of the Muslim woman and, therefore, they did not stipulate any color. Rather, Shaykh Ibn ‘Uthaymeen said, "It is allowed for the Muslim woman to wear what she wishes of clothes of any color except what is considered to be Tabarruj (improper display of beauty and adornment)."
For more information, please, refer to fatwas 87398, 28482 and 87813.
Assuming that the female Companions did not wear except black, this does not necessarily entail that it is prohibited for the Muslim women to wear other colors. The color of the clothes and its design is not a matter of worship that requires imitation of the early Muslim generation.
Allah knows best.
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