Methos was, once upon a time, Death on a Horse, one of the Four Horsemen of the Apocalypse. In a very disturbing fashion, he wore all white, rode a white horse, and killed people. And enjoyed himself immensely while doing so.
Then he got bored and decided to stop doing that.
So let us say in his wandering hither and yon, he stumbles across the quaint little kingdom of Valdemar
which is ruled by a king but protected by the Heralds. Who wear white. And ride white horses (aka Companions). And generally try to avoid killing people when at all possible although it's not out of the question. But even with that difference in attitude, you might see a reason for Methos to have a few rather nasty flashbacks.
He tries to stay away from the Heralds whenever possible. He's decided he likes brown horses. Brown is a good color. Doesn't mean much of anything to anyone, really. It's not black, white, red, or green (and I'm fairly sure that's just an odd translation in my bible, but it does say that pestilence rode a green horse, I kid you not. In another translation, it says sickly, but who rides a sickly animal into battle?) Anyway...
Heralds are actually chosen by their Companions. The Companions live out in Companion Field near the royal palace. When they come of age, they go off questing for their Herald, their rider, with whom they form a psychic connection. Sometimes they just pick someone because the personality match is there and the person is a good and virtuous individual. Other times, they pick someone because, in addition to all of the above, this individual has some skill or ability that is or will be needed for the protection of the kingdom.
So I can imagine a time when Valdemar is having trouble with some great empire and needs a clever diplomat and politician. And who better than Methos?
So Methos is confronted by a white horse who says, come, wear white and ride on me...
Methos takes one look and sprints the other way.
The Companion gives chance and finally manages to corner him, which at least gets a proper answer even if the answer is a firm and final "No."
The Companion persists. It's for the good of the kingdom, for the good of mankind, to help me. Please? And Methos, who does love horses and loved his beautiful white horses from way back when who were still nowhere near as lovely as the Companion, agrees to help. He just knows he's going to regret that.
He still refuses to wear white, though. His first few outfits were quickly stained in the laundry and as fast as the magic bleaches his Companion white, it still can't keep up with the fanciful designs Methos paints on him.
And the good Heralds of Valdemar all have to get used to dealing with someone who's a bit more cynical than they had previously thought possible, but who still has the Companion seal of approval for being good and virtuous underneath it all.