Summer has 3 white hooves and they are tough as nails, in the farrier's words "these are a farrier's dream feet!" My other mare has 4 black feet and they are terrible! Amazing how many people still make comments about Summer's white feet though along the lines of "oh you'll have trouble with those" etc lol. My one would be that colours are strongly linked with temperament. I think it's just confirmation bias, people see a chestnut acting nutty and go "oh yep mad chestnuts" but if they see a grey horse being silly they ignore it. I've seen naughty horses in every colour and IME most of it is to do with how they have been trained. I remember reading an old pony club manual that said bay horses were best because they were steady and black/piebald were the worst because they were mean... Um what?