But real answer, she, and the rest of those people were always light skinned.
Of course they always were, there's absolutely no tone difference between Rose and Ed in the manga. Whereas Scar was always noticeably darker.
It's just that during the original production, they never actually had an official color pallete for anything, so since they were presumsed to be in a desert town, the animators just gave them dark skin since they all live in the desert.
Not so much a presumption as a deliberate deviation. During the course of the first anime they've literally turned the place into a second Ishbal.
Am I the only one who thinks that the explaination of the backstory is done a little too fast? Didn't they push the contents of like 3-4 episodes into this episode 2?
The content itself comes from many episodes, but there's still plenty of information not covered. What they mostly wanted to do is show the event in chronological order and depict the changes to the Gate of Alchemy and introduce Truth.
Complete with references to "all is one, one is all" and Gluttony that new viewers wouldn't get because the event is shown too early.
Heck, this early on they're already throwing phrases like "Xerxes", "Philosopher of the East" and "Rentanjutsu" at us. In some ways it's a really nice way to unify the start of the series with the elements introduced later, in other ways it's just spoiling too much.