Flag system is many times where the most frustration lies for new players, when they get killed and they don't know why. It's a system, that describes what consequences your actions against other players and NPCs will have. You can view your own flag colour to the left of your name, and view other players by putting your crosshair over them. There are five flags in the game that you should get familliar with before you start playing.

Fledgling Yellow - Those are new players that just started their new character. You'll stay fledgling for couple of hours. While fledgling other players can't hit you or steal from you. Be careful tho, if you make a criminal act, you will go grey and all your fledgling protection will be gone.

Innocent Blue - This is the normal status in the game. If negative acts are made against a blue player, the other player will become a criminal. Blue players are protected by guards in cities, can be resurrected by any priest in the world and if you die, looting your corpse will mark other players as criminal. Any criminal act, even if accidental, will mark you as criminal. This goes for any NPC as well, so be aware of blue animals left behind by other players. If you get killed, you can report your killer as murderer, increasing his murder count.

Criminal Grey - When you attack an innocent player or get caught stealing you become a criminal. The flag will last for 2 minutes since your last criminal act, and then it will revert back to innocent. While you're marked as a criminal anyone can attack you without a penalty. Also in cities the guards can be called upon you to kill you on sight and not every priest will want to resurrect you if dead. Aiding any criminal will flag you grey also, so don't go around healing thieves and murderers.

Murderer Red - When you kill an innocent player, they can report you for it and it will give you murder count. When you gain five murder counts you will turn red and be flagged as murderer. The rules for murderers are the same as for criminals, but the flag lasts for much longer. To lose murder counts you will have to wait for eight hours of ingame time without killing an innocent player for each murder count. In most cases if you see a red player you will want to run, hide or beg for your life, altho most of the time they will just kill you for their fun. Not all red players are so called "player killers" (PK or RPK) and not everyone is randomly killing other players, some just go red from PVPing, roleplaying, etc... So as a new player you sometimes have more chances to play it nice, since if their intention is to kill you for no reason, you most likely won't be able to do anything about it.

Guild Mate Green - When your in a guild, all other guild members and their pets will show up as green. This means you are able to attack and loot them without becoming a criminal yourself.

Hidden Criminal Grey with blue stripe - This is a special flag that only you and the person you made a criminal act to (or the other way around) can see. When you duel someone, snoop or steal from someone, and no one else notices, only the person involved with such action will be able to see that you did something wrong. While someone is a hidden criminal, others still see him as innocent. To reveal such person to others, you need to push him and that will flag him as criminal to everyone. Be careful tho, beneficial acts, like healing, against hidden criminal will mark you a criminal even tho you saw him as innocent, so sometimes this can get a bit tricky. It's safer to not do such dangerous things while in city or place with a lot of players.

I should add one more "flag". Since there is no player groups in MO, playing together can get tricky if you accidentally hit someone. There is an exception to that. If you're in the same guild as another player that you hit, you won't be marked as criminal. For that reason I would suggest you get into the same guild as people you're playing with.

Flags are probably the most important aspect of MO when you're starting out, so be aware of them! Always "confirm" what you're attacking or healing, always be sure you're not looting blue corpse (there is even a message in loot window to warn you) and don't loot stolen or illegal items (red border in inventory) if you want to be safe. In highly populated places it's much better to be safe, while outside in wilderness your flag rarely matters, since you won't be meeting players as often and it's easily to wait 2 minutes for grey flag to go away.