Tarkir

Some of this information has NOT been confirmed (to my knowledge). But it looks like its true.

Each clan fought primarily against a dragon and its brood. Each clan respected a, lets call it virtue, of their respective dragon enemy. When the Khans of Tarkir set first came out, it seemed like all Takir dragons shared these 'virtues'. Each clan (in Khans of Tarkir) is a wedge. A wedge, in mtg, is a color combination of a color and its 2 enemies. The clans were based around a single color, and one of that color's allies and one of its enemies. In the mana cost of the wedge cards, the color the clan is primarily based around is to the left. The middle color is the enemy of both remaining colors to emphasize in Triple Khans draft (which aren't really being done anymore), its much more flexible to go into 2 enemy colors first instead of 2 allied colors first. The last color is simply the remaining color. The legendary dragon's that lead the broods are the same color combination as the clan they oppose except they are missing the enemy color.

Clans of Tarkir
Abzan Houses Endurance	Jeskai Way Cunning	Sultai Brood Ruthlessness	Mardu Horde Speed	Temur Frontier Savagery