The King in the frame story of the Waystone has as his colors blue and white:
The door of the Waystone opened and a sudden gust of wind made the lamplight flicker. Two soldiers came in, hunched against the weather, their swords sticking out like tails behind them. Dark spatters of rain spotted the fabric of their blue-and-white tabards. - WMF, ch 136
Which just happens to be the same colors as:
They wore the Maer’s colors but beneath their sapphire and ivory were functional breastplates with steel rings and leather. Each man wore a long sword and a long knife. They eyed me seriously as I approached. - WMF, ch 54
The only way for the current King to wear the Maer’s colors is if: (1) he is the Maer, or: (2) he’s related to the Maer… or Meluan Lackless.
The term Penitent King is important, IMO, because that word is only used in one other instance in the whole series (emphasis mine):
Grey-robed priests followed along beside the wagon, ringing bells and chanting. Many of them wore the heavy iron chains of penitent priests. The sound of the voices and the bells, the chanting and the chains mingled to make a sort of music. All eyes were for Tehlu. No one saw me standing in the shadows of the doorway. - NotW, ch 22
To be both a priest (which the Maer is not) and a King, who wears the Maer's colors, they need to be related to the Maer. It's not his wife, or you'd hear about the Queen. And since we don't hear about the Queen putting a bounty on Kvothe's head, Meluan is likely dead, too.
But, Meluan does have another living relative:
“Actually he’s sixteenth in the peerage,” Sim said matter-of-factly. “You’ve got the royal family, the prince regents, Maer Alveron, Duchess Samista, Aculeus and Meluan Lackless….” He trailed off under Manet’s glare. - NotW, ch 43
So, that means Aculeus Lackless has to currently be a penitent priest. The feud with Ambrose is all a red herring. By the end of it, Kvothe will gladly kill Ambrose. I think he's tricked (probably by Ambrose as he tries to become King) into killing the Maer by being led to believe he's killing Ambrose for harming Auri or Denna (the only ones who would push him over the edge) and kills the Maer (who is King at that time), making Aculeus Lackless now King.