Skin color changes after sports. I can tan in a half hour of sports. Why is that?

Normal variant. Some people tan very quickly; others never tan at all; and most of us are somewhere in between. It's a normal variation among individuals. The fact that you tan so fast is a good thing, in that your body is reacting more quickly than normal in an effort to protect your skin from sun damage, thereby reducing your risk of skin cancer, among other things. But you should still apply a sunscreen daily.