Blue: You can not attack or be attacked by others.
White: You can attack other players with PK-enabled mode.
Rose: Players who end up attacking (even accidentally through aid to PKs) another player will be named Rosa for a while.
Red: Players who beat other players will be named Red for a long period. The more players are defeated, the longer that your name will be in red.
There are several safe areas (cities, caves, and spawn points) where PK mode is disabled. You will not be able to attack other players while not leaving the secure area.
During the battles in cities or in duels, the penalties for PK are temporarily disabled, and you can attack or kill other players without being punished.