Something is a "horse of another color" or a "horse of a different color" if it is entirely different than what was originally supposed. The term is often used to describe new information that causes a complete redirection in thinking. Maria, in Shakespeare's Twelfth Night (Act 2, Scene 3) says, "My purpose is, indeed, a horse of that colour." She's not talking about horses at all, but how she plans to deceive Malvolio. If she had an entirely different purpose, it would be a horse of another color.

All these comments are 100% wrong - Twelfth Night clearly refers to a horse of the same colour. The idiom is a horse of a DIFFERENT colour and arises in the royal court of the Prince Regent/George IV and references a practical joke HRH played on his butler Edmund (yes like Blackadder) Nagle. HRH had Nagle's black horse painted white too fool his butler into thinking he'd gifted him a Hanoverian stud, when it was in fact Edmund's own horse painted white. The colour was washed off in the rain - and all the courtiers laughed at the prank stating - "It's a horse of a different colour!" - see Life magazine Feb 27 1939, Florizel's Folly by John Ashton, and Rees Gronow's memoirs of his life in the court of George IV

Hello Safaa, "a horse of another color " means another matter altogether or a totally different situation ." In your example it means that the fact that this pretty girl is coming with her brother makes it a totally different matter than if it was with her fiance. He probably likes the girl and the fact that the guy is not her fiance makes it possible for him to approach her. Another example : " Gambling is not the same as investing in the stock market. It's a horse of a different color."

In this context it means: This is a totally different situation. Or That changes the situation

"That's a horse of a different color" = "Well, that changes the situation." From "The Wizard of Oz": "We want to see the Wizard." "Nobody sees the Wizard!" "But this is Dorothy - and she's wearing the ruby slippers!" "Well, bust my buttons! So she is! That's a horse of a different color!. Come on in!"