Ill take a guess at it...someone correct me if im wrong, but when you get hard, the skin on your dick stretches to accommodate the extra length. If your skin is naturally a little dark, stretched out it is still the same color as the rest of your skin to some extent, but when you go soft again, it contracts again making the skin layers thicker and more compressed, so naturally, more of something dark rather than a little dark everywhere, would seem darker. Paler people wouldnt have so much darkness because more of something light isnt quite a contrast as more of something dark. (I know im confusing in explanations, and tend to say half as much with double the words, but just bear with me)