Many shades of skin colour in humans are due to

The traits which are not so distinct in their occurrence and are spread across a gradient
are generally controlled by three or more genes and are thus called polygenic traits. 

Besides the involvement of multiple genes polygenic inheritance also takes into account the influence of environment. Human skin colour is another classic example for this. 

In a polygenic trait the phenotype reflects the contribution of each allele, i.e., the effect of each allele is additive.