What does it have with skin color? Just limit the sun for two or three days until you tan, no sunburns after that. It's not like Thais are black. Compare:
https://pixabay.com/p-199674/?no_redirect
https://tonycavanagh.files.wordpress.com/2011/08/5005.jpg
Hardly a big difference. For genetic modification, the best solution is to increase range of tanning rather than changing the constant. It's already possible to get black skin [0] with eg. melanotan-2, so it's not that big of a change.