A horse of a different colour is used to mark a sudden change of subject, or to point out that something just raised is really a separate issue from whatever was being discussed before. It often appears when someone wants to stop a comparison in its tracks: two things might look alike on the surface, but one speaker insists that the second case is nothing like the first. It can also simply flag a switch in topic, as when a conversation moves from one matter to something quite unrelated.
What’s the origin of the phrase ‘horse of a different colour’?
The phrase is often traced to Shakespeare’s Twelfth Night, where Maria says, “My purpose is, indeed, a horse of that colour.” But that line meant close to the opposite of the modern idiom: Maria was saying that her plan was indeed the very thing being discussed, something like that, not something different. Somewhere between then and now the sense flipped, and the saying came to mean the reverse of Maria’s point, something else entirely rather than the same thing. Even those who credit Shakespeare with the phrase tend to hedge that the idiom may already have existed before the play, with Shakespeare simply playing on a saying his audience already knew rather than inventing it outright.
The horse itself has long served English speakers as a general measuring stick for comparison, in the way “strong as a horse” sets an animal up as a standard to be matched or contradicted. That tradition stretches back centuries, and the colour of a horse offered an easy, visible way to say that two things did not match after all.
The specifically American spelling, with “color”, is hard to pin down with any certainty. The earliest dated printed example that can be found is from 1908, in Harold MacGrath’s novel The Lure of the Mask, though this rests on a weak, crowd-sourced citation rather than a verified first use, and no earlier dated instance of the exact wording has turned up.
Much of the phrase’s staying power in the twentieth century owes something to the 1939 film The Wizard of Oz, whose Emerald City carriage ride features a horse that literally changes colour before the travellers’ eyes, giving audiences a vivid, literal picture to hang the old saying on.