Although I do not believe that all humans descended from Adam and Eve, the existence of different races or skin colors doesn't actually refute the possibility. (However, the wide variety of different genetic alleles does rule it out, unless either Adam and Eve lived over a million years ago, or there was at some time in the past, a much higher level of mutations, yet paradoxically these did not lead to higher levels of deformities, which would require constant genetic interference from God.)
A population will get darker just by moving to a place like Africa, as paler people get more skin cancer, sunburns keep them from feeling horny, etc. And a population will get lighter in a cloudy place as lack of Vitamin D (which sunlight allows lighter people to make) causes higher death rates among the dark. This is of course Darwinian evolution. And indeed, the Finns were a relatively dark Middle Eastern people 1,000 years ago, when they first migrated to Finland.