Ok, allow me to say it in simpler terms so you can understand. This is COSPLAY, aka "COSTUME play", where you incarnate a character with a costume, a wig and MAKE-UP. It's a hobby, it's innocent, the main goal is not to offend, but to one enjoy himself or herself by dressing up as a fictional character they like.
So in the end, if you decide to dress up as a dude wearing a pink skirt and who has blue skin, you wear a pink skirt and paint your skin blue. If you decide to cosplay as a dudette wearing a jacket and who has a darker skin than yours, you wear a jacket and put make-up on to make your skin darker. It's as simple as that. I don't see how people make a fuss about it, especially with cosplayers who are just doing their own thing!
And also: since, according to you, white people can't cosplay characters with darker skin, are you also saying that who are considered "fat" can't cosplay characters who are skinnier than them?
PS: If you do decide to answer me, do it through PM so we can both spare this user's comment box.