Craig Judd (Shiro Kuro): Green Ryuujin are the default type. Their stories are to do with travel, exploration, and wonder.
Blue Ryuujin focus on character drama. Perhaps all of the characters are related somehow.
Red Ryuujin focus on combat, war, monster hunting, and more traditional quests.
Black Ryuujin focus on tragedy, betrayal, and darker themes.
The Ryuujin partly act as indicators of playstyle. They also hold an item (or two) which signifies special rules in effect for the session, and have various thematic powers that can affect the progress of the game.