A different color for each nickname
It colors nicknames automatically, so it's easier to spot the same person in the chat and the user list.
Good to know
The same nickname always gets the same color family. If someone changes their nickname, their color may change too, and different nicknames can get the same color.
Colors are darkened on light backgrounds and lightened on dark backgrounds. They don't apply to nickname images, and a color doesn't mean a role or rank.