You've just remindede of something with black and tan. Don't know how common this was but pub I worked in late 80s all the beer you pulled off each pump to clean the pipes and it was a significant amount as there were 3 bars and a function room, all went back in the keg for that beer. All except for the Guinness cos it had its own gas in I think, you could blow your head off if you messed with a Guinness keg. So the Guinness went in the mild. All the old boys raved about the Mild in this pub it got talked about. They were all drinking black and tan.