I admit it's a common belief! I'm sure I've even said it myself... And I have a perception that part of what makes Cyn's feet so hard is that they are dark black. Bay is black on one side front and back, and white on the other side. But the farrier says the white and dark feet on the same horse are no different from each other.
But it might also be the power of suggestion... White feet ~look~ softer.
I'd think if she always throws the same shoe, then she's overstriking or stepping on it with one of her other feet.
I've heard this debate for years too. I had someone, I can't remember who, say that the type of hoofs like Bay has, mixed are the best. Black can get too hard and break/crack easy, white is too soft and doesn't hold shoes well, but mixed is more balanced:)
One of those theories that can't be "proved", IMO.
Becky
It may just be that foot . . . regardless of its color. She does tend to throw a superficial crack in one of her front feet- the farrier had to put a staple in it once but I started her on Source as well as her glucosamine/chondroitin/MSM and that seems to have cleared that up.