What causes the differences in physical characteristics like hair color among people?

In genetics, the physical characteristics are referred as phenotype. Each character such as hair color in offspring is determined by a pair of genes.  One gene is inherited from father and other gene is inherited from mother. The phenotype is determined by the dominant gene. The recessive gene of a pair does not show its effect in heterozygous condition. Therefore, the offspring inherits a dominant gene and a recessive gene then the dominant gene will show the character. This is called Mendel’s law of dominance.