i think the color diff is hot...my cousin has got it real nice: his head plus a about an inch down is a really nice light caramel, well almost beige color, while the rest of his hot ten incher is almost chocolate brown
The human body varies in many wonderful ways. If you look around you'll see lots of pics of guys who dicks are almost like those old two-tone cars, occasionally you'll see one that looks spotted. I'm sort of turned on by the guys who are essentially white but who have a very dark cock. To quote Forest Gump (for probably the first and last time I'll ever do that) "life's a box of chocolates...". Something for everyone - enjoy!
Ditto with nips - you got your big old pink ones, big dark ones, tiny little ones that stick up like pencil erasers - all are fun in their own way.
before puberty, your penis is light skinned. During pubert, most (but not all) males see their penis ger darker. Melanin does that. (same reaction as when you are under the sun, but the melamin is "on" permanently. Some areas of your penis may have less of that and this be lighter.
Exposure to sun may darken the skin of your penis (aka: spend time at nudist beach). But it is not sure if it will increase contrast between the darker and lighter portions, or even it out.
The top of mine is just about perfectly even in color. The bottom though is kind of like a spotted dick. I didn't like it at first but now I don't mind, if I knew it actually turned people on I might even like it more. These are the only pictures I have that kind of show it. In person the contrast is even more intense.