It is not part of the Prophetic Sunnah to have a special dress for mourning in Islam. Muslim scholars state that singling out a certain type of clothes for offering condolences should be avoided.
Sheikh Muhammad Al-Mukhtar Al-Shinqiti, Director of the Islamic Center of South Plains, Lubbock, Texas, states the following: There is no special dress for mourning in Islam, and any exaggeration in showing grief is surely a discouraged act. The Muslim is supposed to practice patience and to show respect to all what Allah decrees. That is why Islam made the condolence period only three days so that the people would not continue to live in sorrow and grief.
As for the colour of the mourning garment, there is no specific text to define it. Therefore, I do not like that Muslims follow the custom of non-Muslims in wearing black; neither do I recommend that they do the opposite and wear white. But they should wear the normal clothes that they got used to in normal days.