that’s a horse of a different color字面意思是颜色不同的马，说明跟其他马是不一样的，那实际意思是表达：完全是另外一回事，另外一码事。

【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。

Why didn’t you say that in the first place? That’s a horse of a different color!
你们为什么不早说呀？那完全是另一码事！