Ooooooh there are soooooo many reasons for it! When it comes to coat colour there are many, many genes which contribute to the final markings....you have genes which encode the colour and as said before you could have a dog which carries genes for all possible colours but due to the dominant/recessive issue it turns out one single solid colour. You can also get co-dominance were two alleles (a allele is a version of the gene) are both equally dominant....this has to do mostly with different 'shades' of colour.
You also get genes which encode 'transport' proteins which take the pigments to the hair.....the recessive allele for this is competely defective so if a dog has this it will be white (there isn't a white pigment gene, the white colour in dogs is due to the pigment not getting to the hair cell).
Also all genes are not 'turned on' all the time, they are carefully controlled by the products of yet more genes throughout development, so a gene which 'turns' on the gene for eyebrow hair (this is longer and harder than the rest of the fur) might also turn on a different allele for coat pigment than the gene which controls the rest of the fur.
Also............ not all cells in the body contain exactly the same genetic info, there are lots of little changes which can happen when the cell is copied (all cells in the body come from the single complete cell made when the sperm and egg fuse), if these changes happen inside the coat genes early in development then the animal is what we call a genetic mosiac, this is the pattern we see is blue roans etc.
Oh I just love the genetics of dogs.....I read an interesting journal about dog size and the IGF2 gene.......but I tend to bore people with my science talk so I will leave that for another day!
