The expression “a horse of another color” means something of a different nature from that under consideration.
Just how long it has been that this phrase has had such a meaning is anyone’s guess.
It was known by Shakespeare, though he used it as “a horse of the same color” when describing the plot hatched up by Sir Andrew Aguecheek and Sir Toby Belch, in Twelfth Night, to get Malvolio in Dutch with his mistress, Olivia.
Shakespeare didn’t explain the meaning, so it must have been well known to his audiences, but there is no earlier record of its use.
The probabilities are that the expression was a natural statement, possibly made by some favorite princess at a tournament or a race. She may have thought that her favored knight or horse was losing, when, seeing otherwise by better view, she exclaimed, “Oh, but that is a horse of another color!”
Delighted courtiers would have repeated the saying on all occasions. Or, perhaps, the saying may have been derived from the archeological mystery, the celebrated White Horse of Berkshire. This is a crudely delineated figure, on an enormous scale, of a galloping horse, excavated in the chalk of a hill in western Berkshire.
It is 374 feet long, covering about two acres of ground. Legend attributes the figure to a commemoration of the victory of King Ethelred and his brother Alfred (later, Alfred the Great) over the Danes in 871, but the figure is unmistakably of much greater antiquity.
It would have become undistinguishable from the surrounding terrain many centuries ago, however, were it not for the custom of the people of the neighborhood to make it “a horse of different color” periodically by cleaning out the grass and debris from the trenches by which the figure is outlined.