I read somewhere about some research indicating that solids, specificly red/golds had the most challenging temper. That seems to be the most popular theory I've come across. People always make jokes about my red cocker, asking if she is especially feisty
And she kind of fits the stereotype, being somewhat hyper and nervous by nature. Don't know how scientific the research was because I didn't really read too much about it. But it wouldn't surprise me if the genes that code for color could also affect other things, such as trainability and energy level. All dogs started out looking the same, and as we bred them to get the temper and abilities we wanted, the appearance started changing too. And then of course we started breeding for looks and that's where it all went wrong, but the appearance started changing before we started obsessing over looks, when most dogs were still bred for their usefullness. I saw a documentary where they tried domesticating foxes by choosing to breed the friendliest individuals. And surprisingly the appearance changed over the genereations even though they bred for temper not looks. Interesting stuff really