a horse of a different color
US /ə hɔrs əv ə ˈdɪfərənt ˈkʌlər/
UK /ə hɔrs əv ə ˈdɪfərənt ˈkʌlər/
1.
a matter or topic that is entirely separate or different from the one being discussed
:
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Talking about our budget is one thing, but discussing a new expansion is a horse of a different color.
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I agree with you on that point, but his personal life is a horse of a different color.