I actually know a few "100% Han Chinese" people who have slightly brown/reddish hair. At first I thought they were dyed, but eventually I was convinced that they were not.
In retrospect, I'm not really that surprised, considering China being continental, and the amount of trade/migration over the centuries.
The same trait occurs in Iran & Afghanistan too. Not sure why this link's on HN though?
as guessed before reading the article, they are indeed the uygur people.