I've seen some horses that are white, or part white and it looks like their vaginas are pink. What breed are those?
It isn't a function of breed, it's a function of color. Doesn't matter at all what the breed is.
If the color pattern continues across the vulva, and the color is dark, the vulva is *PROBABLY* dark. If white, almost certainly pink. Or, if there's a border connecting two colors that crosses the vulva, part may be dark and part pink. We've got one molly (female mule) here on the place that's mostly white except for a darker splat that sort of "wraps around" the left cheek of her butt - sorta looks like she rubbed her butt against wet brown paint - The right side of her vulva is pink, the other side, where the "splat" is, is jet black.
Similar on the other end - A horse with white in his/her color pattern that has white crossing the eye will usually have a blue, or sometimes, part blue, part brown eye. Any other color, probably a brown eye. This is independent of which side the eye is on - If one eye sits in a dark patch, it's probably going to be brown. If the other eye sits in a white patch, it's very likely to be blue, AKA a "watch eye" 'cause it often looks a bit like somebody opened a pocket watch and replaced his eye with it. Likewise for males and their genitalia - If their sheath is in a dark patch, they'll probably have a black penis, if white, probably pink, and sometimes, especially in paints/pintos, the color layouts may put multiple colors across the sheath, which can make a penis that's "splattered" both black and pink. Appaloosas can be interesting - The ones that "show" they're appies in their coat patterns frequently have "chickenpox" on their hairless parts (vulva or penis in particular, often, but not always, their nose) with the "base color" usually light, and spatters of dark freckles. But even appies usually follow the "if they have a blob of color instead of white, the bare skin will probably be dark, otherwise its probably pink" quasi-rule.