Why People have skin color between fair and dark?
Mendel's laws were very basic to aid in the comprehension of genetics.  You think of Mendel's genes as an +- pair where + is dominant and - is recessive, making our +- gene produce offspring with the + trait but still being a carrier of the - trait.

However, when it comes skin colour, eye colour, hair colour, height, tendency to muscle growth, and a thousand other things, the gene is more like a series of ++-++-+++-, + is more recessive than -, but some of + will still show up over -.

In short, the colour of a person's skin is not a simple +- gene of dark and fair; everyone carries a certain amount of pigment genes and certain number of non-pigment genes.  The amount that one carries of each of the genes will determine his skin colour and his/her ability to pass on the dark/light characteristic.