Is it permissible to wear the niqab in any color, or is it restricted to black, based on the statement of the Prophet's wife, may Allah bless him and grant him peace: "We used to go out like black ravens during prayer"?

No specific color is required for a woman's hijab or niqab. The saying "as if there were crows on their heads" indicates that they used to cover themselves with black, but this does not mean that black is a required color, as it has been proven that they also used to wear green head coverings.