I've just never encountered it as... well, what would you call it, it's not quite a saying as such - a kind of cliché, maybe? Let's say it seems to have a higher symbolic value in Britain than it has in Germany, for whatever reason. (Which is funny, because linguistically, Germany is often described as somewhat obsessed with the scatological.)

ETA: From what I've gathered on the topic, the reason for the change in colour is mostly the dogs' diet - apparently dogs were fed a diet much higher in calcium up till maybe the early eighties or so. Bones etc. Apparently that caused the white coating on the shit.