Coat color in degus is usually agouti, meaning the hairs are striped brown and grey at the tip with grey underfur. This color has evolved in degus to act as natural camouflage in their wild habitat. Two of the first color variations bred were 'black' (very dark brown) and 'blue' (light grey). This is likely to have arisen due to a mutation in the color gene turning off either the brown or grey portion of their coat color. These colors originated in Germany and the Netherlands, and are spreading across Europe (in particular Sweden and Finland) due to their novelty.
Before you read on, you might like to know a little bit about color genetics. Coat color is expressed in one of two forms; homozygous or heterozygous. The term 'homozygous' means that degu has two copies of the same allele (one half of the neucleotide coding that makes up the whole gene) to make up the color gene. On the other hand, a 'heterozygous' degu has one allele corresponding to one color and one corresponding to a different color. The allele that gets expressed in the phenotype (outward appearance) is the one that is dominant over the other (the other is termed 'recessive'). A good example of this is human hair color- if a person with homozygous brown hair produces a child with a person with homozygous blond hair, the child will be born with brown hair no matter if they're homozygous or heterozygous for the brown color, since brown hair is dominant over blond hair.