Volleyball: why does one player wear a different color?
I've been agonizing over this question for a while. I used to play a lot of competitive volleyball when I was younger, but I never understood this recent development in the game. It turns out that the person with the different color is a "libero."

The libero is a player specialized in defensive skills: the libero must wear a contrasting jersey color from his or her teammates and cannot block or attack the ball when it is entirely above net height. When the ball is not in play, the libero can replace any back-row player, without prior notice to the officials.

This replacement does not count against the substitution limit each team is allowed per set, although the libero may be replaced only by the player whom they replaced.

The libero may function as a setter only under certain restrictions. If she/he makes an overhand set, she/he must be standing behind (and not stepping on) the 3-meter line; otherwise, the ball cannot be attacked above the net in front of the 3-meter line. An underhand pass is allowed from any part of the court.

You can read the full answer here.