If Adam and Eve were the first humans on earth, the scriptures did not tell us the color of their skin. Hence we do not know by the books whether they were black or brown or white. We can only say that they had a complete genetic constitution and that variation and natural selection occurred mostly due to environmental factors when their offspring spread across earth.
Scientifically however from archeology and anthropology, the first modern humans evolved from earlier forms and they were considerably dark skinned. Leading to the theory that evolution has its roots in Africa. So as humans began to spread and move up north, their skin started becoming paler due to lesser sunlight in those areas hence lesser melanin. This has since become an evolutionary adaptation.