I have a question: my penis is black when I take a shower. It stays black but every hour it changes color to different brown or black shades. Why is that?
The color of your skin is determined by several things. There is the color of the skin cells themselves, but also the amount of pigment that present in those cells. Exposure to the sun causes the skin to produce more pigments; a lack of exposure causes little to no pigments to be produced. Therefore, let's say you keep your groin region covered, as you ought, then you will find the skin of your groin a light shade of color than, say, your arms which are exposed to the sun more often.
The palms of your hands and the soles of your feet have little color because there are few pigmented cells in those areas.
Skin is not opaque. You can see through it a bit. The thicker it is, the less you see through it. Thus, compare the top of your arms with the underside and you will notice that is it lighter in color. Now, the skin of your penis is very transparent because it is very thin. But that means when when your penis is getting more blood because of an erection, arousal, or just because you are hot (as in the shower), then the penis is going to take on a darker red tint. In the same way, your face darkens when you blush.
But your penis is constantly changing size, which varies the thickness of the skin and, thus, the color you see it to be.
Therefore, added all together, your observations are correct. Your skin changes color all over your body, but it is more noticeable in the skin of your penis.