My childhood cocker was golden and was the gentlest dog that ever lived. My three blue roans, Milo and Dylan were/are lovely and kind dogs, and the rescue Finn was a nightmare, but I think he was a bit mental. I had heard the red rage syndrome, but haven,t personally come across it.
I saw that documentary about the foxes, it was very interesting. IIRC they were being bred for fur, and they wanted a calmer temperament, but as they bred for this the foxes became a bit like dogs, with smaller teeth etc.
I don't know if colour makes any difference, I don't think it does in Scotties which is the other breed I have experience with.
Lesley and Dylan