a horse of another colour

horse of another color

Something completely different or separate, especially in comparison to something else. I've always found math to be easy, but calculus is a horse of a another color.
See also: another, color, horse, of

a horse of another (or different) colour

a thing significantly different.
1975 Sam Selvon Moses Ascending Two or three is okay, but when you start bringing in a battalion, it is a horse of a different colour.
See also: another, colour, horse, of