"A horse of a different color" is an idiom meaning a matter, situation, or issue is entirely different, unrelated, or a completely new, separate subject from the one previously discussed. It signifies a shift in context or perspective, often highlighting a contradiction or a significant change in circumstances, such as "that's a different story".
In contrast, "My purpose is, indeed, a horse of that colour" is a line from Shakespeare’s Twelfth Night, meaning "That is exactly the plan" or "That aligns with my intention". It signifies agreement with a previous statement or a shared understanding of a plot, likely foreshadowing the idiom "a horse of a different colour".
- From online with editing
----------------------------------------
Thinking of the coming year of the horse, I started looking for horse related idioms, and am happy to see many. 
Actually Shakespeare was also fond of horses! The Bard of Avon frequently featured horses as symbols of power, speed, and status, most famously in Richard III with the desperate cry, "A horse, a horse, my kingdom for a horse!", which was an APAD we discussed about two years ago (see: APAD: My kingdom for a horse! 2024-04-07).
But, today's APAD is a horse of a different colour from "My kingdom for a horse"! 
