Whether is blonde and black hair has been first designed in the complex composition of the body (the parents). Humans get our hair the color of the parent as an inheritance or a derivative.
Designed? Yes. According to the existing draft of this, a certain hair color was obtained. In the hair structure, there is a section called root hairs embedded (hidden) under the skin. At the root of the hair, there are among other cells that affect or create a color, called melanin. (Same with substances that affect skin color, check here).
This melanin into the hair when the hair growth process. And hair can get more colors (dark) or otherwise get less color (fading) rather than the substance melanin itself. The more melanin that ‘flow’ of the hair, the darker the hair color, in other words, the black hair color. While melanin is only a few will make the lighter colored hair or a blond. These colors (black, brown and blond) is a normal color in human hair.
What about the red hair (natural)? There is no certainty about this. But there are many allegations that the secondary dye is mixed with normal pigment melanin. Another suggestion is the change in the normal dye (black, brown and blond) who may be affected by external factors.
Gray or white appearance to the hair, has a similar explanation. Dye on the hair root cells were decreased (usually due to aging), the less absorbed into the hair. Indeed a growing number of incoming air, thus making the hair look shiny gray to white.