Take a character concept
Write up the character as you would any other
At some point write the words "dark skin" in the appearance section of the CS
???
Profit

No, but seriously, the majority of the time skin colour is not going to make a lick of difference, much in the same way that in most roleplays biological sex is only going to have situational differences, because most people will treat others the same regardless of those differences. In A.M.R.O. especially, because they only care about two things; "can you use alchemy?" and "do you work for us?". It only really becomes a character-changing attribute in a setting that focuses on those attributes (so skin colour may become an extremely important part of the character in a roleplay set around the American Civil War).

...of course, there's a little bit more to it than that, but it's the simplest way of handling this stuff.