Dueling
You can duel any player of any class or level. Just click on the person and right click on his/her name. Click on the Duel button and wait for the other person to accept. Wait for the 3-second countdown and start fighting.

Player Kill
Player Killing or PK mode can be activated once you reach lv30.

Level 1-29: Blue Name: PK Protected Status. Cannot attack and cannot be attacked by other players
Level 30: White Name: PK Status. Can attack and can be attacked by other players with same status (Dying from PK will not cause loss of experience). You can turn your PK status off or on. Just click on the PK Button shown above.
PK Color Code of Character Names:
Blue: Protected. Cannot attack and cannot be attacked by others
White: Can attack players in PK status.
Pink: Players who have recently attacked another player will their name turn Pink for awhile.
Red: Players who have killed another player will have their name turn red for a long period of time. The more players killed, the longer the character’s name will remain red.
* Light Red
* Red
* Strong Red
NOTE:
- When your name is Red you will lose more experience when killed by a monster and blue named players will be unable to cast support magic on you. Other players’ names will not change color if they attack you.
- You cannot support blue and white named players when in pink mode.
- You cannot shift to safe mode when your name is red.
- If your name is red and you kill monster that are of equal level or higher level, the red status will disappear faster.
- There are many safe zones in Perfect World. PK is not allowed in these zones.
- There will be city battles and some special instances where players will be forced into a PK status. In times like these, no punishment for PK will be given.
- NPC Guards will attack a red-named person in towns.
- Items also have a high chance of being dropped when your name is red.

Source: ph-wiki, ampota.com, asiapworld.com