Ok, going on what I've read and seen of Sarkhan Vol so far, he originally rode/fought with the Mardu Horde but had a falling out by learning the shamanistic ways of the Temur seers, the first time we see Sarkhan in Alara he is Red/Green, the next time we meet him in Zendikar he is Red/Black, already pointing at the signs of Jund, but with his heritage of Mardu being Red/White/Black, he is currently sitting at 4/5 of the color pie, now perhaps working with both the Temur and for Nicol Bolas has possibly ingrained some sort of blue magic within him. I know that being affiliated with the Mardu and the Temur doesn't particularly mean he has those colors associated with his powers. Perhaps in Fate Reforged or in "Louie" we will have the possibility of seeing a pure five color planeswalker.
That said, I wouldn't mind a 5C Sliver walker though.
UR Melek, Izzet ParagonUR, B Shirei, Shizo's CaretakerB, R Jaya Ballard, Task MageR,RW Tajic, Blade of the LegionRW, UB Lazav, Dimir MastermindUB, UB Circu, Dimir LobotomistUB, RWU Zedruu the GreatheartedRWU, GUBThe MimeoplasmGUB, UGExperiment Kraj UG, WDarien, King of KjeldorW, BMarrow-GnawerB, WBGKarador, Ghost ChieftainWBG, UTeferi, Temporal ArchmageU, GWUDerevi, Empyrial TacticianGWU, RDaretti, Scrap SavantR, UTalrand, Sky SummonerU, GEzuri, Renegade LeaderG, WUBRGReaper KingWUBRG, RGXenagos, God of RevelsRG, CKozilek, Butcher of TruthC, WUBRGGeneral TazriWUBRG, GTitania, Protector of ArgothG