[efeX]

As lavamike said, you can't have 2 colors.

But to make it a tag just say

new name[MAX_PLAYER_NAME], str[MAX_PLAYER_NAME+3]; // str[MAX_PLAYER_NAME+4] is if you want the tag like you said [M] PlayerNameHere
GetPlayerName(playerid, name, sizeof(name));
format(str, sizeof(str), "[M] %s", name);