"But before kennel clubs I think all of the above were a mixture of colours, most often white with patches (of brindle, black, red or tan) and sometimes black, brindle, red or fawn. This is what we see with old paintings (and some old photos) depicting the working ancestors of these breeds regardless of country. "

I have held this belief for many years now.  You are spot on in my opinion in that the show breeding and exhibitions drove the evolution of the colors as the clubs and judges expressed their preferences and anointed "champions'> Much the same way that some are trying to change the Caucasian Ovcharks and other of the LGDs where it is more about the size, color, and coat than about the ability to guard.

I prefer a white masked dog but in my chosen breed that is not easy to find anymore. Maybe one day we will have a hard reset and go back to function over form.