Another matter entirely from the subject at hand, something else.
The phrase may have originated with real horses. Pure-breds are registered at birth and the information on the registrations includes their color. When a horse is sold, the registration is also transferred. Occasionally, the color recorded on the registration doesn’t match the actual color of the horse. Horses’ coats do sometimes change color as the animals age, but I’d be inclined to suspect that the horse being sold is not the one represented on the registration but is an entirely different horse.
The saying has been around since at least the 1600s. Shakespeare wrote, in Act II, Scene 3 of Twelfth Night, this dialogue by Maria: “My purpose is, indeed, a horse of that colour.” It’s possible that the phrase evolved from that speech but my guess is that the idiom already existed and Shakespeare was twisting it.
I have read that knights in medieval tournaments rode different-colored horses in races so that spectators could tell which knight to root for. Gambling has always been a favourite pastime and so it is reasonable enough to believe that those who lost bets in tournaments would be told of their loss with news that a “horse of a different color” was victorious.
Or is that stretching it a bit? Well, I wasn’t around in medieval times, so I can’t say.
Here’s a fun example of the phrase in use: In the 1939 movie, “The Wizard of Oz,” Dorothy and Toto along with the Strawman, The Tin Man, and the Cowardly Lion, find themselves at the gates of the Emerald City. They experience difficulty convincing the Guardian of the Emerald City Gates to let them in. When they finally persuade him to let them in, he says:
“Well, bust my buttons! Why didn’t you say that in the first place? That’s a horse of a different color! Come on in!”
Once the group is inside, the next scene shows the group in a horse-drawn carriage and the horse, of course, changes colour from shot to shot. Dorothy remarks to the driver, “I’ve never seen a horse like that before!” And the driver responds, “No, and never will again, I fancy. There’s only one of him, and he’s it. He’s the Horse of a Different Color you’ve heard tell about.”