Horse of a Different Color

It means 'quite a different matter'.

Easy!

Let's see, we can make a simple conversation.

Frank : Hi, what are you doing now?

Michael : Oh, hi. I'm making a plan to go to Japan.

F : Wow, when are you going to leave?

M : I'm not sure, I'm just making it.

F : Oh, Michael. I think it's not a productive activity.
Making a plan and Going to the abroad is a horse of a different color.

M : I know that, thanks.

Hmm. If you said like Frank, you would be a nosy.