Why do Thais have blacks and whites?

Thais are black and white, and many people in northern and northeastern Thailand are still relatively white (close to China). Personally, I feel that the sun in Thailand does not hurt people, so Thais are not afraid of the sun. Therefore, it is easier to get a tan. The sun in China is very painful, so I hide indoors. If you live in Thailand for a year or so, it will be very dark.