You know....I feel that "hey, thats how god made us". And other cosplayers may be able to loose weight, dye their hair or other things but theres nothing you can do about being born a different race. I say cosplay whoever you want!
However I kinda agree with Lolita_Tobi somewhat. Sometimes to make it, I feel people with darker skintones should sometimes tweak the cosplay.
For example...I'm african american and have cosplayed Misa from Deathnote once, insted of getting a high yellow blonde wig I purchased a more muted brownish blonde. It matched my skintone more, and seemed more natural, yet I was still in character.
xD I hope I made sense here ~