Well, first of all The Color Purple falls under the broadest genre of fiction. The three primary genres are poetry, drama, and fiction.
Hair color is determined by two different forms of pigment: eumelanin (which is the largest influence) and phenomelanin. Eumelanin is a black pigment, and phenomelanin is a red or yellow pigment. Hair color is determined by three factors having to do with pigments: how much pigment is present, to what degree a eumelanin or phenomelanin is present and how close together the melanin (pigment) granules are. The more eumelanin a person has, the darker his hair will be.
The most common gene which controls the color of our hair is a brown/blonde gene, which consists of a dominant brown allele and a recessive blonde allele. A person with a sequence of mostly brown alleles will have brown hair, whilst a person with no brown alleles in their DNA sequence will have blonde hair. Several genes control the dark shades versus light shades and give a person their exact shade of hair color. It is a totally random process which alleles a baby receives and explains why you are highly unlikely to have the exact same shade of hair color as any siblings.