The numbers of each genotype of mice in the questions above shows that there are more dark mice living on the dark substrate and more light mice living on the light colored substrate. But when we consider comparative overall fitness for the entire area, which fur color seems to have the greatest overall selective advantage, and why

Dark colour.

Explanation:

Dark fur color seems to have the greatest selective advantage because of their hard physical body which occurs due to extreme environmental conditions. The dark colour of mice occurs due to the high intensity of sunlight which increases melanin in their skin. These dark colour mice pass through a very hard and extreme environmental conditions so that's why Dark fur color seems to have the greatest selective advantage.