If I cosplayed America, I would go for a golden color, like a golden blonde. I've never actually cosplayed America, but have been with people who have. If you want to look at different photos of people doing the cosplay, check out this site: http://en.curecos.com/
It's an Asian site, but in English, so not too difficult to navigate. They have large galleries for specific characters, and you can look at the different wig colors and find which works best.