a horse of a different color 完全是另一回事；另当别论 ；不同的事；完全不一样

【例句】
Wow, you mean the guy who sits next to Susan is her husband? I thought they were just friends. Well, that's a horse of a different color.
哇，你的意思是坐在Susan旁边的那个人是她的丈夫？我还以为他们只是朋友哩。这可是两回事。

You mean to say that guy with that beautiful girl is her brother! I thought he was her boy friend. Hey, that's a horse of a different color -- I'll go ask her for her phone number!
你意思是说和那漂亮姑娘在一块儿的是她的兄弟！我原以为那是她的男朋友呢！嗨，这样就情况完全不同了。我可得去问她要电话号码！