Black short for volleyball are not mandatory. However, most manufacturers do not make them in a lot of colors. Besides black, you may see red navy or royal, but not much else. I've seen purple on ECU, but that's about as wild as they get.
As for the jerseys, they aren't really loose. They are typically form fitting. That being said, some players may wear a size up so they aren't tight, but they aren't loose. Volleyball players wear their uniforms tight so they don't get in the net. Loose uniforms can get caught, which could be dangerous. It would also cost them a point.
Even the men wear a more form fitting uniform. It's typically a sleeveless shirt and board shorts, as opposed to baggy basketball shorts.