What are the factors that determine the chances of survival based on skin and eye color in the surviving members of a population?
Humans, like any other species, are subject to limiting factors that affect the survival chances of any given population. Some of those limiting factors are abiotic (non-living). Those things might include availability of water, air quality, weather, natural disasters, temperature, etc. Other limiting factors will be biotic (living), such as natural predator and prey relationships, crowding and stress, war, etc. In short, limiting factors are the availability of food, water, and shelter.
In a pure Darwinian survival of the fittest model, a case might be made that a certain skin color and eye color allow a certain population of humans with those traits to have better/more access to the food/water/shelter. If that is the case, those people are more likely to survive. More likely to survive means a longer period of reproductive viability and more chances to reproduce than those without the higher skin color and eye color fitness. That's why Darwin called it "survival of the fittest." Those that are most fit for a particular environment tend to be the ones that pass on their genes to the next generation, thereby creating a species with a predominant set of genetic characteristics.
Humans, however, aren't so easy to place in a Darwinian model. While it is possible that a certain eye color and skin color is better for survival against limiting factors, those traits might not be socially accepted. The people with those traits, while "fitter," might be considered social outcasts, which would limit the number of potential and willing mates. Determining the chances of survival within a population while being given only two traits to work with is likely too limited in its scope.