Update: I asked the Discord and apparently Zhu Yilong had dyed his hair red for a romcom he filmed before Guardian, and red dye takes forever to get out. At least some of the other actors might have similar things going on.

(Of course, Han Chinese people can and do have brown hair that looks paler/reddish in bright sunlight due to this guy called Genghis Khan and his paler-haired-than-the-Han Mongols conquering all the places ever.)