Why is my skin so strange.?

Hello im a 15 year old guy and my i cannot understand my skin. It is fairly white on the chest , stomach, legs, front thighs, and upper back. But my rear neck, Area around Butt, and some rear lower back are VERY Black. My dick and balls are also very black. This is strange. Please help. If someone needs more description, I can send pics to your email. Please suggest ways to fix it.