That's not really an of course... There's several Planeswalkers for each color, not even counting the variants on them they make. There's at least two versions of Liliana (Black), Ajani (White), Chandra (Red), Jace (Blue), and Garruk (Green) each. Then there's Tezzeret (Blue), Nicol Bolas (Blue, Black, Red), Tibalt the Fiend Blooded (Red), Tamiyo the Moon Sage (Blue) and several more...
Tibalt, by the way, is the first planeswalker to cost ONLY two red mana to summon and his strongest power allows the player to take control of ALL creatures for the turn, untap them, AND give them all haste.