the origin of skin colors which are inheritable.
There really is only one skin colour; brown. This is caused by melanin and depending on how much we have we can vary from white (little melanin) to black (lots of melanin). This is controlled by several genes giving about 64 shades of brown.
The theory that best fits the facts is that Adam and Eve were created with 2 alleles for each of these genes so they had the genetic potential for all skin colours. Their children would have had a variety of skin colours. Some black, some white, some in between. Assuming random pairing in this generation some couples would have had both black, both white, or some mixture. This would then have created families where some had skin colour predominately at both ends of the range.
So how did we get races with different skin colours? Perhaps an event that divided the population into family groups, some white, some black, most in between. The Tower of Babel meets that requirement. The new languages groups would have kept families together and locked skin colours into language groups. As these groups spread out they took their new languages and skin colours with them.
Selection and drift could then have acted to reduce variation within each group.