okay so those that i have encountered so far are:
Red quests - which are very important usually to acces a new area like UW,Fay go, Misty isalands, yeti islands.
Yellow are normal quests.
Green are repeatable.
All of those are with ! in them.
When accepted they turn into gray with ? in them. Those just remind that you have taken a quest from the NPC you are talking to.
Greed with a clock on them are event time only available quests.
Blue with 2 gears on them is used for annualy avaible buffs. like Ullerni, Black dragons, Free BoE drink.
Its also used for repeatable actions like destroying sepulcher coins
http://warofdragons.com/artifact_info.php?artikul_id=26662 .