Were the wives of the Messenger of Allah, may Allah bless him and grant him peace, not wearing anything but black, and is it permissible for a woman to wear the hijab in other colors?

There is no evidence to suggest that the wives of the Messenger of Allah, may Allah bless him and grant him peace, wore only black. Rather, there are narrations indicating that some of the female Companions wore black. It is narrated from Umm Salamah and Aishah, may Allah be pleased with them both, that when the verse of the Hijab was revealed, the women of the Ansar came out as if there were crows on their heads, due to the coverings they wore. There is no harm for a woman in wearing black or any other color, for the important aspect is modesty (covering), not the color or the style.