It is very strongly related to culture and perception of other cultures, but mostly its random.
There are lots of clichés like black men prefer white women, white men prefer asian women, etc.. Those are stereotypes but they could show tendencies.
Those tendencies are basically random as there is no real, objective reason for that.
Some native central americans thought the whites that arrived were gods. As in their mythology gods often were pale skinned - then actually seeing pale skinned people blew their minds.
Also: Exotic is exciting. There is always a special place for exotic things in people's minds. Same goes for exotic partners.
The evolutionary perspective
What is the benefit of preferring other skin colours / other melanin amounts? While you're in your area you've been genetically adapted to for hundreds of generations?
None, your skin colour is most likely better for this enviroment.
BUT people from vastly different enviroments have a significant amount of other minor adaptions. So they bring new stuff to the gene pool. That is a benefit for the entirety of the population.
All you can say for sure is that the more exotic varieties will get some special attention - which can be dampened or enhanced by cultural influence.
Examples:
- Most traditionalist places have the general public being rather uninterested in other cultures and people. In Japan there is a strong preference of other japanese people. It is a very conservative place and they have one of the lowest refugee numbers of all first world countries. Far below even the US.
- In arabic countries you will not see that many couples with black and non-black people. The arabic word for "black man" literally means or comes from the word "slave". A strong preference for black people is pretty unlikely in that area.
Conclusion
There is no way of predicting the development of random preferences in general.