Can you substitute capers for green peppercorn? I'm making a sauce for a roast.
Capers are generally very salty, and not what you want as a peppercorn substitute.
I think that black/white/red/madagascar peppercorn would be a fine substitute. The distinction between the different pepper flavours is very subtle, and not one to worry about much.
Capers and peppercorns are completely different. I am not saying that you cannot substitute one for the other, it is just you won't get the same final result. Since it appears you are looking for "Green Peppercorns" they are very mild compared to Black ones. If you have white pepper or any other peppercorns other than black you would be OK. You can always just leave it out too.