Answer: The mother and the father are both heterozygous brown-eyed (Bb).
Explanation: Both parents are brown eyed because they both have a heterozygous allele (Bb) for brown eye. The heterozygous allele has one gene for brown eye (B) which is dominant over the recessive gene for blue eye (b). So when the two heterozygous brown eye individuals cross, there is a chance of having a blue eyed child (bb).
See the attached punnet square for more illustration.
Grey Mice have a higher fitness because their phenotype allows them to blend in with their environment, thus helping them to avoid predators. This allows them to survive and reproduce better than the other mice.