Milk hurts when it goes out your nose.
Snow and water are quite reflective of UV, so I would think that nude snowboarders might also have a cause for concern.
Maybe the coloring is due to more capillaries being located there. I mean, isn’t that why our lips are pink - because there’s more capillaries and more blood flow to that region than in the surrounding skin?
Last I looked (which was longer ago than you’re thinking), my bunghole wasn’t brown so much as a darkish pink. But I’m very fair - perhaps it differs with skin tone.