Why do women wear black clothes during Hajj, while men wear white clothes?

A woman in a state of ihram may wear any color she wishes of lawful clothing, as long as she adheres to the Islamic regulations, and does not wear a niqab or gloves.

As for a man in a state of ihram, it is recommended for him to wear white, though he may wear other colors if needed, and he is prohibited from wearing sewn garments.