Melanin:
It is a skin pigment occurs in human body. Melanin content of the body has a key role in the skin color, and is what makes hair, skin, and eyes appear darker. If a person has high concentration of melanin in the body will have darker skin and the vice-a-versa.
Racism:
This is the geographical location that plays an important role in skin color. Racism is another responsible factor for changing skin color. The US people have a fair complexion, whereas Asian has a darker complexion due to the geographical factor. Since the US beauty ideal has been predominated worldwide, this becomes an ideal to get fair skin for a beautiful face.
Sun exposure:
Excessive sun-exposure induces melanin production results in darker skin.