Some mutant genes are able to 'mask' another gene. This is not the same as dominance where a mutant allele located at a particular locus can 'override' a corresponding gene at the same locus on a sister chromosome. Masking is where a gene at one locus on a chromosome can hide the effects of another gene at an entirely different locus on the chromosome.
For example a mutant gene which does not allow the black pigmentation to show in the Syrian Hamster (the Black Eyed Cream gene) can mask a mutant black gene. That is, the chromosomes could contain two genes for a black hamster but if the chromosomes also contain two genes at a different locus that prevents black pigmentation from showing, the resulting offspring will be genetically both black and cream but will have the appearance of a cream despite them having two black genes as the cream genes mask the black genes appearance.