Human skin has red, yellow, brown and blue tone; the skin tones result from combination of complex bio-molecules such as melanin (brown pigment), beta-carotene (yellow to orange pigment), hemoglobin (red pigment) and the condition of the skin.
The most prominent of which is melanin, produced by melanocytes under the stratum basale. Some areas of the skin simply have less melanin than other areas. As the cells containing more melanin come in contact with prolonged period of sunlight, they become darker. Other areas with less melanin or that have less exposure to the sun, remain lighter. This gives the skin an appearance of an uneven skin tone, or even pigmentation.