Do you have a link to back up your claim of 600,000 Americans living in China? That sounds a bit high. I would also like to know how many of them are Caucasian. I know from experience that when you're a white guy in China, you may speak some Chinese (I don't), but everybody will see you as a foreigner.
I don't know about the exact demographics, it can be inaccurate often and who will prove census wrong?
I visit China around 10 times a year and I'e seen more brown people there rather than white people, and Chinese treat all of them the same. They are rude and isolated and don't care about other people, I've even seen racist Chinese peeps there, so idk but yeah.